Update 12.50pm: Railway stations have reopened after a gas pipe leak in central London was repaired.
Engineers located the gas escape at the junction of Craven Street with Corner House Street, and are now checking nearby buildings.
People who made it into the PwC offices on Craven Street on Tuesday morning have since been evacuated, and a police officer said the gas level is still "extremely high".
Another officer told a crowd of workers that it could be up to four hours before they will be allowed in.
Charing Cross and Waterloo East stations were shut, with services diverted into Victoria, Cannon Street and Blackfriars.
Both stations reopened before noon, with disruption expected to continue for the rest of the day.
✅Charing Cross underground station has now reopened. https://t.co/QKlg7YZvx2— TfL travel (@TfLTravelAlerts) January 23, 2018
Update 10.41am: Gas pipe repaired after leak leads to evacuation in central London
A gas pipe has been repaired after a leak led to the closure of two major railway stations and the evacuation of 1,450 people in central London.
Heaven nightclub was cleared and guests at a nearby hotel were moved into emergency accommodation after the alarm was raised in the Strand at around 2am on Tuesday.
Engineers located the gas escape at the junction of Craven Street with Corner House Street, and are now checking nearby buildings for gas.
Gas supplier Cadent said: "A repair on the gas pipe has been completed.
"We are now checking the closest properties to ensure there is no gas in these buildings.
"No-one can enter these properties until we have completed these safety checks."
Cadent said the emergency services' cordon will remain in place until all safety checks have been completed.
"Our priority is to get the train stations opened as soon as possible, followed by the buildings.
"Engineers are working to resolve the situation as quickly and safely as possible and we hope to have the stations and streets open by late morning," the company said.
Earlier 10.04am: Evacuation underway in central London after discovery of gas leak
Hotels and businesses near the Strand in central London are being evacuated as a precaution following the discovery of a gas leak, UK police said.
The Metropolitan Police said officers and the London Fire Brigade were working with the National Grid and other agencies at the scene.
Members of public have been told to keep away as a cordon has been put in place and motorists have been advised to avoid the area as a number of roads have been closed.
A London Fire Brigade spokesman said: "We are assisting police on the Strand after a ruptured gas main was discovered using detection equipment and high levels of natural gas were detected in the atmosphere.
#CharingCross station is closed & #Strand is shut off due to high levels of natural gas coming off of a ruptured gas main in Craven Street, WC2. Please avoid the area & find alternate travel routes https://t.co/BstLLR9N7Q pic.twitter.com/dAjOBWKpRm— London Fire Brigade (@LondonFire) January 23, 2018
"As a precaution, approximately 1,450 people have been evacuated. They are from a hotel and a night club.
"The Strand is completely closed and we are asking people to avoid the area.
"We do not know the cause of the gas leak at this stage. It is ongoing and engineers are down at the scene trying to isolate the leak. There are still high readings of natural gas in the area.
"We are working to sort it out as quickly as possible."
It is the second time in recent weeks that hundreds of people have been evacuated in London due to a gas leak following an incident on Horseferry Road in Westminster on January 9.
On Tuesday, Cadent Gas said engineers were on site on Craven Street behind Charing Cross station to try to locate the gas escape.
A spokesman added: "We are working with the emergency services and a safety precaution an exclusion zone has been put in place.
"The Amba Hotel, nearby flats, offices and Charing Cross stations were evacuated. Craven Street and the Strand from Duncannon Street junction to Trafalgar Square is closed."
The leak is understood to be on Craven Street.
Brett Loft, LFB spokesman and station manager at Chelsea, said he could not be sure how long the roads will remain closed, saying it depends on how long it takes gas authorities to find and seal the leak.
He said: "It has happened a few times recently.
"With the ageing pipes in London there is an ongoing programme at the moment to replace all the pipework.
"It’s just one of those things. It happens."
Mr Loft said there is no danger to the public.
Due to gas leak in the #Strand, WC2 businesses and hotels are being evacuated as a precaution.— MPS Westminster (@MPSWestminster) January 23, 2018
Matthew Wiggins, 30, a software tester from north London, was one of the workers who arrived at the cordon.
"If there’s a gas leak I’d rather not go and stand near it.
"I’m going to work from home which is frustrating because all my stuff is in the office," he said.
A police officer at the end of Craven Street has told a group of workers - many from PWC - that it could be 15 minutes or four hours before they will be allowed in.
We continue to assist gas engineers and other partner agencies at the scene of a gas leak on #Strand. Two fire engines and two fire rescue units remain at the scene standing by https://t.co/BstLLR9N7Q pic.twitter.com/YH3kq5qThA— London Fire Brigade (@LondonFire) January 23, 2018