David Cameron’s attempt to reach out to Labour voters in his conference speech has suffered a humiliating blow after it emerged that the man he used as an example of someone the Tories managed to convert at May’s election has been voting for the party since the 1980s.

The Prime Minister proudly boasted how he had won over Bernard Harris, an 82-year-old from Leicester who had written a letter to him before polling day explaining that he had once “foolishly voted Labour” but had concluded that “only a Conservative government” could achieve what he wanted.

Mr Harris told the Mirror that it had in fact been Margaret Thatcher who convinced him to vote Tory in 1987 and not Mr Cameron in 2015.

He has not voted for any other party for the last three decades, he said.

In his speech to the Conservative party conference – his first since the election – Mr Cameron made a clear pitch for voters in the centre ground as he set out his intention to exploit Labour’s shift to the left under Jeremy Corbyn.

He said it was “never too late” to defect to the Tories and to help his cause he recalled the letter he had received before May’s election.

He told party activists: “Bernard Harris from Leicester wrote to me before polling day and said this. Aged 82, this is possibly my last election: ‘In my life I have foolishly voted Labour, believing it served the working class. How wrong I was. Labour is against all I aspire to.

“I am 100 per cent for a United Kingdom, a sound economy, free enterprise, a trading Europe and a decent standard of living.

"Only a Conservative Government will achieve this."

Mr Cameron added: “Bernard, you found the right party – and I want many more to follow in your footsteps.”

Mr Harris told the Mirror he was not pre-warned that his letter would be used in Mr Cameron’s speech.. "They asked me for permission to use the letter, but didn't tell me it was for the Prime Minister,” he said.

"I'm a humble person and I was happy for them to use it, because I meant every word I said."

Asked whether Mr Corbyn could convince him to return to voting Labour, he said: "I don't doubt he's a very sincere man, and believes what he says. But I think he's misguided. Mostly over the nuclear deterrent.