Mr Lenihan’s move is likely to convince a bloc of his allies to do likewise and throw their support behind Mr Cowen.
It could spell the end of Mr Martin’s hopes to become party leader before the election — and allow Mr Lenihan time to prepare his rival leadership bid in a post-election scenario.
Mr Cowen’s camp was confidently predicting victory last night, having already received the endorsement of a number of cabinet ministers and TDs and knowing Mr Lenihan’s was still to come.
One source said Mr Lenihan’s intervention would “have the effect of alienating Martin and his side”.
Mr Lenihan is believed to have a core group of about 12 TDs who will take their cue from him.
Transport Minister Noel Dempsey became the latest cabinet figure last night to announce he would support the Taoiseach.
By contrast, Tourism Minister Mary Hanafin, seen as a key ally of Mr Martin, refused to say how she would vote, leaving the Foreign Affairs Minister with few senior figures publicly declaring for him.
Despite the momentum appearing to swing in the Taoiseach’s favour, Mr Martin insisted the vote was going to be tight and he could defeat the motion of confidence in Mr Cowen.
In an interview with the Irish Examiner last night, Mr Martin said a change of leadership was urgently required to give Fianna Fáil a chance of having a “critical mass” in the next Dáil.
“It’s about giving the party a chance to campaign effectively in the election so that we have a significant parliamentary party in the next Dáil,” he said.
Mr Lenihan is expected to make his announcement after today’s cabinet meeting and before TDs cast a secret ballot on the motion of confidence.
The Fianna Fáil parliamentary party — its TDs and senators — will gather around 5.30pm in Leinster House to begin debating the motion. Only TDs will have a vote, however.
The party has 72 TDs but Ceann Comhairle Seamus Kirk will not vote and neither will Justice Minister Dermot Ahern as he is recovering from a hip operation, meaning a total electorate of 70.
Fianna Fáil has said 36 votes will be required to pass or defeat the motion.
However, the party was unclear last night what would happen in the event of a tie at 35 votes apiece.
Party chairman John Browne, general secretary Sean Dorgan and Government Chief Whip John Curran will meet this morning to draw up the rules for the vote in the event of a tie.
They will also decide on two tellers for the vote, who will be “elder statesmen” of the party and acceptable to both sides.
Each camp made a last-ditch attempt to shore up support, with Mr Cowen saying he would “go out and fight for the party’s honour” in the election while Mr Martin said “the very future of the party is at stake”.
Despite the party dropping to just 14% support in the latest opinion poll, Mr Cowen said he is still the right person to lead it.
“We’re in a fight and we need a fighter to lead Fianna Fáil into the general election, to put the Fianna Fáil record straight. And I want to lead that fight. I think I’m equipped to do it.”
If he survives as leader, it’s understood Mr Cowen will retain Mr Martin in his cabinet for the sake of “unity” ahead of the election.