The audience of 10,000 women representing the backbone of Britain responded first with sporadic heckling, then slow handclapping and finally undisguised jeering from sections of the hall.
If it had not been for an appeal for politeness from the chair, the prime minister might not have been able to scamper through to the end of an ill-advised speech that later drew taunts in the Commons from William Hague.
"It is the mark of an out of touch prime minister that you don't know why you're out of touch," the Tory leader said.
Mr Blair's mistake was to use the fiercely non-party-political WI as the backdrop for making political points that might have appeared almost bland in another setting.
There was no evidence at the outset that the women were hostile to Mr Blair or to Labour. They gave a warm welcome to the father of four, fresh from paternity leave, and seemed more flattered than offended by his suggestion that they were "the most terrifying audience I have seen".
The institute prides itself on its ability to move on from the jam-making image of its founders in 1915. To the speechwriters in Downing Street it must have seemed the ideal example of blending tradition and modernity - the recipe Mr Blair now wants to present as Labour's political vision for Britain.
But yesterday it was the prime minister who got into a jam of his own making and ended in a pickle when he ignored advice to be non-political that was given when Downing Street approached the WI last week, requesting an opportunity for him to speak. The trouble started half way through the speech, with heckling from part of the hall where West Midlands delegates were clustered.
Mr Blair had been trying to defuse the row between the chancellor, Gordon Brown, and Oxbridge colleges over university admissions policies. "Let's hear no more about class war ... We are unashamed supporters of excellence," he said.
But the audience reacted badly when he started on a list of examples of government action to extend opportunity. He began with the unemployed and sport in schools. Then came the disabled, lone parents and childcare. As he moved through internet access, the minimum wage and family tax credits, the heckling started.
Members later described it as a party political broadcast.
Mr Blair appeared not to understand what the problem was as he ploughed on about NHS reform. As the slow handclapping grew in volume, Helen Carey, the WI's chair, appealed for calm.
The penny seemed to drop after he was heckled during references to cuts in interest rates since Labour took power in 1997. Forced off his stride, he said: "Well, I'm glad we're having a good debate anyway." Women responded with tut-tutting that this was a party lecture not a debate.
Mr Blair managed a brief recovery. He won applause for references to children turning up at school in the morning, fathers taking responsibility for their children's upbringing and paying women the same as men for the same job. But there were more shouts when he returned to the political language of collective responsibility, and sounds of incredulity when he gave support to the WI's campaign to save rural post offices.
At the end there was polite applause from perhaps as many as half the women present. Others sat impassively.
"When he asked last week for time on our agenda he was told we are a non-political organisation," said the institute's spokeswoman, Sangeeta Haindl. "Our chair warned that if he tried to make it political he would get short shrift."
Alastair Campbell, Mr Blair's spokesman, told reporters after Mr Hague and Charles Kennedy had exploited the PM's embarrassment at Commons questions: "I have no intention of getting into a war of words with the WI."
What the WI hated...
List of government initiatives (heckled)
Explanation of plans for NHS reform (slow handclap)
Interest rates cut since last election (heckled)
Mention of collective responsibility (cries of No, No)
Effect of technology on rural postal services (heckled)
... and what it liked
Fathers should help to bring children up
Defaulting fathers should lose driving licences
Benefit claimants should attend job interviews
Equal pay for women doing same job as men
Help for women who stay at home with pre-school children
National Fed. Women's Institutes