It was a day like any other campaign day. Then Mrs Duffy took to the stage

A grandmother who popped out to buy a loaf of bread and bumped into the Prime Minister yesterday found herself the unwitting principal in an encounter that has already become one of the defining moments of the 2010 general election campaign.

The meeting with Gillian Duffy wrecked Gordon Brown's preparations for today's televised leaders' debate and threatens to haunt him to polling day – and beyond. Pictures of the humbled Prime Minister arriving at Mrs Duffy's pebble-dashed terraced house in Rochdale, Lancashire, to apologise for calling the lifelong Labour supporter a "bigot", were beamed around the world.

What he said indoors during the 39-minute meeting remains unknown.

The blunder horrified Labour strategists, who had been pinning their hopes on today's debate about economic issues. Particularly damaging appeared to be the juxtaposition between his public statement to the 65-year-old widow – "Very nice to see you" – with his private thoughts seconds later: "That was a disaster...should never have put me with that woman... ridiculous...bigoted woman."

The extraordinary chain of events began when Mrs Duffy spotted a gaggle of camera crews, journalists and Labour apparatchiks surrounding the Prime Minister, who was in the town to watch young offenders clean a cycle path.

Mrs Duffy seized the opportunity to challenge Mr Brown over the national debt, and the Labour candidate Simon Danczuk ushered her over to meet the Prime Minister. Mrs Duffy began by telling him she was "absolutely ashamed of saying I'm Labour", and went on to complain about the scale of the deficit, taxes paid by pensioners and university tuition fees. In passing, she also mentioned "all these eastern Europeans" heading for Britain.

The meeting ended amicably and Mrs Duffy told reporters she was definitely going to vote Labour after all.

However, seconds later Mr Brown's private thoughts – caught on the microphone attached to his lapel – were broadcast on national television, and they were very different from his public platitudes. After he got into his limousine, the Prime Minister snapped at his adviser, Justin Forsyth, that the meeting had been a "disaster" and demanded to know whose idea it had been. As the flustered aide said he did not know, Mr Brown pinned the blame on his long-serving assistant, Sue Nye.

Mrs Duffy was still only a few yards away when astonished reporters caught up with her to relay what her Prime Minister really thought of her.

The clearly upset pensioner replied: "He's going to lead this county and he has called an ordinary woman who has just come up and asked him questions ... and he's calling me a bigot."

Back at Labour headquarters in London it was clear the party had a public relations disaster on its hands. Their horror deepened as Mr Brown was forced to listen in a Manchester studio to a recording of his remarks during an interview with Radio 2's Jeremy Vine.

As the tape played, the Prime Minister, who had obviously forgotten he was being filmed, held his head in his hands. Watching in London, Lord Mandelson, Labour's campaign supremo, ordered his press team to get a message to Mr Brown to warn him he was on film. Too late: the picture of the despairing Labour leader will become an enduring image of the campaign.

The party launched a desperate damage-limitation exercise as soon as he left the studio. The Prime Minister called her at home to apologise, telling her she was "a good woman", while Lord Mandelson went on air to tell the world Mr Brown was "mortified". The Labour high command feared it still had not done enough and dispatched the Prime Minister to Mrs Duffy's home to eat humble pie.

With battalions of journalists outside, Mr Brown spent 39 minutes with the pensioner. Wearing an awkward grin, he emerged to say sorry and insist she had accepted his apology.

Tony Blair's former press secretary, Alastair Campbell, said the Prime Minister had been determined to "atone" for his blunder. "I saw him at his Manchester hotel," Mr Campbell wrote on his blog. "I don't think I have ever seen him so angry with himself ... She was so clearly not a bigot, and he knew that."

Last night Mrs Duffy's niece, also called Gillian, said she did not believe that her aunt would be satisfied with the apology: "He has shown his true colours. He's always trying to pretend to be so nice and in touch with the people, but he's obviously not." Earlier, Mrs Duffy told reporters she had already filled in her postal vote for Labour. It is now unlikely to be sent.

In an email, Mr Brown also apologised last night to activists for the damage to the Labour campaign. Labour strategists admitted preparations for the debate were in disarray. One said: "We fully accept there will be a feeding frenzy over what happened in Rochdale. Gordon will have to address it in the debate."

The final televised leaders' debate will be screened on BBC1 at 8.30pm

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