'With God as my witness...'
A sensational YouGov survey of Labour members and supporters found the veteran backbencher has secured the backing of almost half the electorate.
The scale of the crisis facing the party prompted Mr Blair to warn Labour against a lurch to the left.
Giving a speech this morning, he launched a scathing attack on the socialist leadership hopeful, saying: "People who say their heart is with Corbyn, get a transplant."
Comment: Says the Tin Man!
He said:
"It's like going back to Star Trek or something. Back to the old days. I wouldn't want to win on an old-fashioned leftist platform. Even if I thought it was the route to victory, I wouldn't take it. We won elections when we had an agenda that was driven by values, but informed by modernity."
Comment: The 'values' of modernity: economic debt slavery, illegal wars of aggression, and rampant nihilistic narcissism (led and instigated by pure psychopathy). Who would ever want to abandon such values?
But Mr Blair added that he was "Labour through and through" and would not quit the party if the leftwing candidate succeeded in winning the contest.
In the rare intervention, Mr Blair warned that it was not enough for Labour to be a "platform for protest" against cuts and that it had to develop a credible strategy for government if it wanted to regain power.
"In the next few years there are some very tough things happening in public services, in welfare. People are going to feel very angry, very disenfranchised, very dispirited," he said. "Labour shouldn't despair. We can win again. We can win again next time. But only if our comfort zone is the future and our values are our guide and not our distraction."
Comment: Translation: The international financiers who own this country intend to squeeze the people even harder, and they need full compliance from all political leaders to get the job done.
But he stopped short of pledging his support for any of the other three candidates.
The results have sparked panic among the leading lights of the opposition, with one top aide from the New Labour years suggesting that two of the three more moderate candidates should drop out and back a single anti-Corbyn candidate.
In a blow to bookies' favourite Andy Burnham, Corbyn - Labour's most rebellious MP - is currently polling at a whopping 43 per cent.
The shadow Health Secretary is his closest rival, trailing 17 points behind at 26 per cent.
Senior figures in the party - still bitterly divided from its crushing defeat in May's general election - have slammed the MPs who nominated Corbyn.
John McTernan, a former aide to Tony Blair, told BBC's Newsnight: "They are morons".
Shadow Business secretary Chuka Umunna, deemed a favourite to succeed Ed Miliband until he made a shock withdrawal from the race, will today say that Labour "urgently need to modernise again".
He will add: "Too often over the last five years in opposition we behaved like a party of protest."
Analysis of voters' second preferences shows Corbyn - who struggled to reach the 35 nominations needed to get on the ballot - beating Burnham in the final round with 53 percent of the vote.
Britain's Alexis Tsipras? Jeremy Corbyn, an actual socialist, is leading Labour Party polls to become its next leader.
Liz Kendall, considered by many to be the Blairite candidate, is a distant fourth with 11 per cent.
The news that the 66-year-old anti-war campaigner is set to win the leadership race is likely to shock Labour politicians who only nominated the Islington North MP in order to "broaden the debate".
Comment: Indeed, someone made a mistake there in letting anything remotely resembling a 'voice of the people' from encroaching on the psychopaths' turf.
It was widely expected that the anti-austerity candidate would have no chance of winning, but his hard left politics have proved popular with the Labour faithful.
Corbyn is also leads the race for nominations from local constituency Labour parties, receiving the backing of 74 to Burnham's 73.
The polling bombshell looks set to deepen Labour's woes in a week that has seen the party hit by infighting over £12 billion of Conservative cuts to the benefits bill.
Interim leader Harriet Harman ordered her MPs to abstain - but 48 Labour members, including London mayoral candidates Sadiq Khan and David Lammy defied the party whip to oppose the controversial legislation, which includes cuts to child tax credits.
Although Burnham criticised the Labour's handling of George Osborne's Welfare Bill as "a mess" and said the party was "crying out for leadership", Corbyn was the only one of the four leadership candidates to vote against it.
Corbyn, who has come under intense scrutiny for his links to terrorists from Hamas, Hezbollah and the IRA, is thought to have benefitted from a surge in support from trade union activists - said by the poll to be backing him by a margin of 37 per cent to his closest rival.
Comment: Watch now as the British media collectively character-assassinates Corbyn by 'revealing his dirty laundry'.
The news that a fifth of the 1,054 people surveyed by YouGov are still undecided will be scant consolation to Labour - who were today cheekily replaced as the "official opposition" in the House of Commons by seat-robbing members of the SNP.
Newly-christened frontrunner Corbyn - whose odds have shortened from 100-1 to 2-1 over the past month - is thought to be so unpopular among the wider electorate that even members of the Conservatives have been signing up as Labour supporters in order to vote for him.
Comment: That's an interesting bluff on the Tories' part. Their real fears of Corbyn leading the opposition is betrayed by the fact that the British media has been activated to 'nail him to the wall'.
As for Blair, 'get a heart transplant' is an interesting turn of phrase from the man who pretty much ripped the heart of the old British Labour Party and turned it into a heartless Neo-Liberal, NeoCon supporter of ruthless Western empire.