The Prime Minister was confronted by 66-year-old Gillian Duffy while on the campaign trail in Rochdale.
He spent nearly five minutes answering her questions and told her: "It's been very good to meet you."
He smiled at the woman and then got into a waiting car.
However, a microphone picked up his words to an aide as he drove away.
Mr Brown was caught saying: "That was a disaster."
"You should never have put me with that woman."
"Whose idea was that?"
Mr Brown went on: "It's just ridiculous."
His aide then asked: "What did she say?"
Mr Brown replied: "Oh, everything, she's just a sort of bigoted woman who said she used to vote Labour."
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