Left-winger Jeremy Corbyn is on track for victory in the Labour leadership contest, according to a poll.
Research by YouGov for The Times has found the backbench MP is the first preference for 43% of party supporters - way ahead of bookies' favourite Andy Burnham on 26%.
Shadow home secretary Yvette Cooper was on 20% and Liz Kendall just 11%.
The study also forecast that after Ms Kendall and Mrs Cooper had been eliminated and second preferences redistributed under the Alternative Vote system, Mr Corbyn would beat Mr Burnham 53% to 47% in the final round.
The results, which are likely to fuel a growing sense of panic in Labour's Blairite wing, emerged after Mr Corbyn burnished his credentials with the Left by defying acting leader Harriet Harman to vote against the Government's Welfare Bill.
The other leadership candidates fell into line by backing the Opposition's "reasoned amendment" to the legislation - which was defeated - and then abstaining on whether it should progress to the next Commons stage.
Mr Corbyn, who struggled to find the 35 MP supporters required to feature on the ballot paper, is now the 2-1 second favourite with Ladbrokes to win the race - having previously been as long as 100-1.
He has been nominated by 71 local Labour associations - just one behind Mr Burnham.
John McTernan, a former special adviser to Tony Blair in Downing Street, told the BBC's Newsnight programme the figures were "disastrous".
He suggested two of the frontbenchers should consider dropping out in order to pool centrist support and block Mr Corbyn.
"These figures are disastrous for the Labour Party, disastrous. The fact is the other candidates need to decide who is the ABC candidate - the Anyone But Corbyn candidate," he said.
Mr McTernan also complained that party activists were “suicidally inclined” and condemned MPs who had "lent" their nominations to Mr Corbyn in order broaden the debate, even though they did not want him as leader.
"They need their heads felt," he said. "They are morons."
Mr Blair is due to address an event hosted by the Progress think tank later, and is likely to repeat his warnings against a Labour lurch to the left.
Before the general election he cautioned that when a traditional left-wing party took on a traditional right-wing party it usually ended with a "traditional result" - a Tory victory.
Earlier, Mr Corbyn posed for a photograph as he had coffee with Sinn Fein leaders Gerry Adams and Martin McGuinness at parliament.
Mr Adams tweeted a picture of the meeting, writing: "With Jeremy Corbyn & the comrades @ Portcullis House, Westminster."
Sinn Fein MPs do not take up their seats in the Commons because they refuse to swear an oath of allegiance to the Queen.
Mr Corbyn's links to the Irish republicans have been controversial in the past. He has been criticised for observing a minute's silence in 1987 for the eight IRA members killed by the SAS in Gibraltar.
The YouGov polling also found that another figure on the left of the party, Tom Watson, has a sizeable advantage in the deputy leadership battle.
Mr Watson had 42% of first preferences, ahead of Stella Creasy on 21%, Caroline Flint on 17%, Ben Bradshaw on 11% and Angela Eagle on 10%.