Update: 15.53pm: More than 158 Conservative MPs have publicly declared they will be supporting Theresa May's leadership during tonight's confidence vote.
The tally represents a majority of the party's MPs, and, if all follow through as they have stated they will, it means Mrs May will remain leader of the party and British Prime Minister.
The total was calculated by the Press Association from comments by MPs on social media or in statements to media outlets.
The vote will be held from 6pm to 8pm, and with the restoration of the whip increasing the number of Tory MPs to 316, Mrs May now needs 159 votes - half of the parliamentary party plus one - to secure her position.
Government ministers in the House of Lords issued a statement in support of Mrs May.
The statement read: "The Government front bench team in the House of Lords fully supports the Prime Minister.
"Theresa May is the right person to lead both the country and the Conservative Party and we urge our colleagues in the Commons to vote to support her this evening."
Asked whether Theresa May was confident of winning this evening's vote, a senior Downing Street source said: "She is fighting for every vote. We have seen support from across the parliamentary party this morning but there is a lot more to do this afternoon."
The source added: "This vote isn't about who leads the party into the next election, it is about whether it makes sense to change leader at this point in the Brexit process."
The source said he was "not aware" of any plans by Mrs May to give Tory colleagues a date for her departure as PM or Conservative leader.
He said: "She has said on a number of occasions - in fact she said immediately after the last election in 2017 - that she would serve as long as her colleagues want her to.
"She believes it is her duty to serve as long as the party wants her to."
The Downing Street source said that Mrs May was informed by Sir Graham Brady that the 48-letter threshold had been crossed in a telephone call some time before 11pm on Tuesday.
She had returned to 10 Downing Street at around 9.30pm after a day of travel which took her to The Hague, Berlin and Brussels for talks with EU leaders.
Asked how she was dealing personally with the challenge to her position, the source said: "A lot of people have talked about her resilience in different situations and I think we have seen that again today.
"As ever, she is determined and focused."
European Council president Donald Tusk has said the EU must "discuss the state of preparations for a no-deal" Brexit at Thursday and Friday's EU summit "as time is running out" for a breakthrough, writes Fiachra Ó Cionnaith.
In an open letter to European Council members before the crucial meeting in Brussels this week and as British prime minister Theresa May faces a no-confidence vote this evening, Mr Tusk said focus must now turn to a no-deal crisis.
Outlining the schedule of the two-day meeting this week, Mr Tusk said: "given the seriousness of the situation in the UK, let me start with Brexit".
"The intention is that we will listen to the UK prime minister's assessment, and later, we will meet as 27 [EU member states] to discuss the matter and adopt relevant conclusions.
"As time is running out, we will also discuss the state of preparations for a no-deal scenario," he wrote.
While Mr Tusk's letter is couched in carefully worded diplomatic terms, the fact a potential no-deal Brexit cliff edge in just four months time is the main item on the EU summit agenda underlines the crisis facing the continent.
Previous EU summits in November, October and earlier this year have attempted to find a solution to the stand-off without any long-lasting success, with pressure now mounting on politicians to cut their losses and finalise no deal plans vital to businesses and communities in all nations.
Taoiseach Leo Varadkar has said he still believes that the Withdrawal Agreement will be ratified by Westminster and the European Parliament.
Speaking in the Dáil, he said: "We have a meeting with the European Council on Thursday and Friday and will have an opportunity to engage with Prime Minister May and an opportunity to speak with my colleagues.
"I'll be taking a call with President Juncker later on today to see what assurances we can give the United Kingdom parliament that might assist them to ratify the Withdrawal Agreement.
"However, that cannot be a change in the substance of that agreement including the substance of the protocol on Ireland and Northern Ireland. But that is what we are going to work towards."
Sinn Féin deputy leader Michelle O'Neill said the Irish Government needed to "remain firm" on the border backstop despite the "debacle" at Westminster.
"It is about making sure the backstop is the only show in town, it is the bottom line, it is the insurance policy," she said.
Ms O'Neill said the leadership challenge against Mrs May was the latest twist in a "civil war" engulfing the Conservative Party.
Sinn Féin's Stormont leader said the UK Government was only interested in British interests and said the Irish Government, along with other EU nations, had to protect the interests of the people of Ireland.
An exchange between two Conservative MPs on the BBC News Channel has caught the attention of Brexit-watchers on Twitter.
Live on the Victoria Derbyshire Show on the channel, Ms Derbyshire just finished talking to Tory MP Andrew Bridgen and said she was going to talk to his party colleague James Cleverly, who is vice-chair of the Conservative Party.
She spoke to Mr Bridgen saying: "I gather that you don't necessarily want to talk to each other. Are you happy to?"
Mr Bridgen replied: "I'll go, if that's fine," before leaving.
Victoria Derbyshire introduces James Cleverly: "I gather you two might not want to speak to each other?"
Andrew Bridgen: "That is fine, I'll go."
Bloody hell. pic.twitter.com/U66EpLBnut— John Johnston (@johnjohnstonmi) December 12, 2018
Meanwhile, Democratic Unionist Party MP Sammy Wilson said it was a matter for the Conservatives to choose who was their leader, but that their policy on Brexit must change.
Mr Wilson said in a tweet: "Our confidence and supply agreement is with the Conservative & Unionist Party.
"The current Brexit policy is totally unacceptable to us and the House of Commons. It is entirely a matter for Conservative MPs to decide who their leader is... but the policy must change!"
Sir Graham Brady has said that if Mrs May loses the vote of confidence, the party would hope to elect a successor as quickly as possible.
He said that he believed that balloting among MPs could be concluded before Parliament breaks for the Christmas recess on December 20.
Under party rules, there is a process of "exhaustive" ballots of MPs until all but two leading candidates are eliminated.
They would then go forward to a final postal ballot of the entire party membership.
"I think it is perfectly likely that if there were to be a contest and if there were to be a very large field, as is occasionally speculated, it is entirely possible we could do it in two or three rounds (of voting among MPs)," he told reporters at Westminster.
"I would have thought we want to conclude that process as swiftly as possible and allow the matter to move on to the party in the country to decide.
"We could operate ballots on successive days. If it were to happen our goal would be to conclude the parliamentary stages, if possible, before the Christmas recess."
Mrs May won the backing of the Tory Reform Group, the largest membership group within the party, which represents its liberal wing.
In a statement, the group said: "This is a critical time for our country. It should not be about the Conservative Party, but about our national interest, implementing the decision taken on June 23 and taking our country forward.
"MPs voting tonight must think carefully before casting their ballot. Our party is, and must be, about more than Brexit - our country will not forgive us if we forget that.
"With the very real prospect of a hard-left Corbyn government, now is the time for unity.
"We urge our parliamentary colleagues to support the Prime Minister."
Northern Ireland Secretary Karen Bradley came out in support of the current leader, posting on Facebook: "I will be voting for the Prime Minister, Theresa May in the leadership challenge."
DUP leader Arlene Foster told Today: "I can't say I'm surprised because being here in Westminster yesterday I did realise there were a lot of conversations going on, however my focus, of course, has to continue to be on the Withdrawal Agreement and the fact that the backstop needs to be taken out of that Withdrawal Agreement."
Scotland's First Minister Nicola Sturgeon wrote on Twitter: "Today is a stark reminder that the UK is facing chaos and crisis entirely because of a vicious civil war within the Tory party. What a self-centred bunch they are. They all need to go, not just the PM."
Today is a stark reminder that the UK is facing chaos and crisis entirely because of a vicious civil war within the Tory party. What a self-centred bunch they are. They all need to go, not just the PM.— Nicola Sturgeon (@NicolaSturgeon) December 12, 2018
1922 Committee Chairman Sir Graham Brady said he spoke with Theresa May last night to inform her that the threshold had been reached and a leadership ballot would take place.
"The Prime Minister was very keen that the process should be concluded as quickly as reasonably possible," he told reporters on Wednesday morning.
"She will come at 5 o'clock this afternoon to speak to colleagues in the 1922 committee and then we will conduct the ballot immediately after that, between 6 and 8 o'clock, with a result announced as quickly as we are able.
"It was clear she was keen to have resolution. She wanted to have this process concluded as quickly as possible. I think you can take it she was keen to get on with business."
Sir Graham said it was "not a surprise" to reach the threshold given the months of "endless speculation".
He said Conservative MPs who are unable to be present in Westminster tonight will be able to vote by proxy. "So I think all colleagues will have a fair, open opportunity to participate," he said.
- Digital Desk and PA
Conservative MP Anna Soubry has apologised to the Irish people for the actions of her party and government in the Brexit debacle.
She told RTE’s Morning Ireland that she was “acutely appalled” at the confidence vote in Prime Minister Theresa May.
“Not even the Conservative party could be so politically irresponsible at this critical stage with the most important decision since the end of World War 2.”
The confidence vote is coming from a group of far-right Brexiteers who have “blighted the Conservative party for decades,” she added.
Theresa May “has got to bring this blasted vote back to the House of Commons” and has to have “a people’s vote, to take this mess back to the British people.”
It is “the most irresponsible thing to do to change leader” at this time, she said.
“It won’t make Brexit go away.” There is a danger that the Conservative party could end up with Boris Johnson as leader, she warned.
Ms Soubry said she did not have a problem being a rule taker not a rule maker within the EU “if that is the price we have to pay for peace and British business.”
She described the signatories of the letter calling for a vote of no confidence in Theresa May as being like children. “They can’t get their own way, so they stamp their feet. They need to grow up.
“Let the British people vote. I hope they see sense and that the best deal is the current deal with the EU.”
By Fiachra Ó Cionnaith
British prime minister Theresa May will face a confidence motion in her leadership tonight in a vote that could kill off her time in power and all but guarantee a hard Brexit.
Conservative party rebels triggered the vote after sending in 48 letters seeking her resignation just hours after Ms May was warned by EU leaders there will be no changes to the Brexit backstop deal.
In a statement this morning, the chair of the Conservative party's 1922 committee Graham Brady said the required 48 letters needed to force a vote has been reached.
Confirming the ballot will take place between 6pm and 8pm this evening - the same time as Ms May was due to meet Taoiseach Leo Varadkar in Dublin - Mr Brady said the crucial vote is now imminent.
Ms May, who was told of the imminent vote late last night, is due to give a statement at Downing Street this morning in which she is expected to announce plans to fight on.
If Ms May wins tonight's vote, she will be untouchable for a year.
However, should she lose, it will not only kill off her career but will all but guarantee a hard Brexit and depending on when a British leadership race takes place could delay the March 29, 2019, Brexit divorce date.
The dramatic events in Britain have also called into doubt Ms May's crunch meeting with Taoiseach Leo Varadkar in Dublin this evening.
The meeting was due to take place at Government Buildings before the EU summit in Brussels on Thursday and Friday.
However, even if the meeting still goes ahead, Mr Varadkar is not expected to give Ms May any solace after yesterday saying he and EU leaders will not budge on the existing deal.
In a whistle-stop tour of Europe yesterday, Ms May was repeatedly told by European Commission president Jean Claude Juncker, German Chancellor Angela Merkel and European parliament president Donald Tusk no new deal will be allowed.
During meetings in Brussels, Berlin and other European capitals, the British prime minister was told despite the dire need for a breakthrough the EU will not return to the negotiating table.
Speaking before meeting with Ms May, Mr Juncker said the existing deal "is the only deal possible" and that "there is no room whatsoever for re-negotiation", while Ms Merkel told her counterpart there is "no way" new talks will take place.
And, while Mr Tusk indicated a middle-ground could still be reached, the hard-line stance is set to be repeated by Mr Varadkar during their meeting in Dublin, with a Government spokesperson last night ruling out any compromise with Britain on the deal.
Asked on Tuesday about the mounting stand-off and the growing risk of a no deal Brexit unless Britain backs down on its re-negotiation demands, Mr Varadkar told the Dáil Ireland wants to help Britain to ensure the deal is passed.
However, warning there can be no "compromising on the basic fundamental substance of the backstop", he insisted the only options now on the table for Britain are to back the deal, cancel or delay Brexit, or to crash out with no deal in place.
"It remains in the hands of the United Kingdom to decide that we do not end up in a no-deal scenario. There is the option to revoke Article 50 [the legislation which triggers the March 29, 2019, EU-UK divorce date] and the option to extend Article 50.
The hard-line position has significantly increased the risk of a no deal Brexit which could cause havoc to Ireland as Ms May is facing equal levels of pressure from her own party and the DUP to ensure the Brexit backstop is scrapped.
With no breakthrough in sight, Mr Varadkar said the Government needs to "ramp up" contingency plans for a hard Brexit, with Tánaiste Simon Coveney bringing a detailed memo on the crisis plans to cabinet.
However, despite the memo outlining a series of supports for at risk businesses, extra customs officials on borders and at airports, and emergency legislative changes if a no deal Brexit occurs, the Government has continued to insist it is not planning for a hard border.