The result of the ballot among Labour's 380,000 members was a clear warning to Mr Blair that he could not take the support of his party for granted, and that many activists have reservations about his New Labour "project".
As first predicted in The Independent two weeks ago, the left won four of the six seats representing constituency parties on the NEC. To Mr Blair's embarrassment, the winners included Liz Davies, a former Islington councillor, who was barred as parliamentary candidate in Leeds North East because of her hardline views.
Left-wingers were jubilant when the results were announced during the opening session of Labour's annual conference in Blackpool. Three other members of their Grassroots Alliance were elected to the NEC: Mark Seddon, editor of Tribune newspaper, who topped the poll; Cathy Jamieson, a leading Scottish activist, and Pete Willsman, a former ally of Tony Benn.
The only members of the Blairite Members' First slate to be successful were Michael Cashman, the actor and gay rights campaigner, and Diana Jeuda, an official with Usdaw, the shopworkers' union. The left slate won a total of 342,000 votes, while the Blairites managed only 311,000.
The results, due to be announced on Wednesday, were rushed out last night in an attempt to stop Mr Blair's looming defeat casting a shadow over the conference and his keynote speech tomorrow.
But the left could win another victory on Wednesday, when Dennis Skinner, the left-wing MP for Bolsover, may retain his place on the NEC despite moves by Blairite MPs to replace him.
Privately, Mr Blair's allies admitted he had been given a "bloody nose". One said: 'We pulled out all the stops and we still lost 4-2. There is no point in pretending it's anything other than a bad defeat."
But Mr Blair's aides insisted that he would still enjoy a commanding majority on the 32-member NEC, and that there would be no change in the party's direction.
The Prime Minister sought to defuse criticism of his leadership by holding a question-and-answer session when the conference opened yesterday. But he told the delegates bluntly that the party had won its landslide victory last year because he had modernised it. "We forget why we won, and we go back to square one again," he said.
Mr Blair warned his internal party critics: "The choice you've got is not between the Labour government of your dreams and the Labour government you've got. The choice is between the Labour government you've got and a Tory government."
He told the four left-wingers not to use their new NEC as a platform to attack the Government. "What I say to people is 'critical support fine, but if people go into outright opposition that doesn't help anyone'."
"It is a vote to keep the party Labour," said Ken Livingstone, the MP for Brent East. "Most of those who ran as New Labour candidates have not won. It shows that people in the party want to increase tax, spend more on the welfare state, want interest rates to come down and don't want to get into bed with Paddy Ashdown."
Ms Davies said she and her fellow left-wingers had won against the odds. They had been massively outspent by the Blairite candidates and had been "smeared" by senior party officials.
"This will send a clear message to the Labour leadership," Ms Davies said. "Party members want an open, democratic party, where dissent is valued and where voices of the grassroots are listened to with respect."
Full conference reports
pages 8 and 9