Bourne MP Quentin Davies has defected to the Labour Party, it has just been announced.
Mr Davies had been a Conservative MP for almost 20 years and his local supporters were planning a party to mark the anniversary.
Don Fisher, a Conservative Party stalwart who sits on Bourne Town Council and recently stood down from South Kesteven District Council, said: "I am astounded. I can't believe it.
"I have known Quentin since before he became an MP. I was just about to write out a cheque for his party and Iain Duncan-Smith was invited as a guest. That will have to be cancelled now."
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Mr Fisher was speaking from the local Conservative office, having heard the news on the radio.
He added: "Everybody is stunned."
Mr Davies resigned from the Tory party in a letter to leader David Cameron.
He wrote that under Mr Cameron the party "appears to have ceased collectively to believe in anything, or to stand for anything".
He added: "Because my constituents, to whose interests of course I remain devoted, are entitled to know the full background, I am releasing this letter to the press."
His defection comes on the eve of Gordon Brown becoming Prime Minister.
Mr Davies (63) is a former diplomat and has been shadow Northern Ireland secretary and shadow defence secretary. He became an MP in 1987.
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