Tory MP Quentin Davies has defected to Labour, "delighting" new leader and prime minister-in-waiting Gordon Brown.
The MP for Grantham and Stamford made his decision public in a letter to Conservative leader David Cameron, with whom he has long been at odds.
He wrote that the party seemed "to have ceased collectively to believe in anything, or to stand for anything".
Mr Cameron wrote back to Mr Davies saying: "You have made your choice and the British people will make theirs."
The defection comes the day before Mr Brown takes over as prime minister from Tony Blair.
Mr Davies, a pro-European, voted for former chancellor Ken Clarke in the Tory leadership contest which Mr Cameron won in 2005.
In his letter, he wrote: "Under your leadership the Conservative Party appears to me to have ceased collectively to believe in anything, or to stand for anything.
"It has no bedrock. It exists on shifting sands. A sense of mission has been replaced by a PR agenda."
Mr Davies added: "Although you have many positive qualities you have three, superficiality, unreliability and an apparent lack of any clear convictions, which in my view ought to exclude you from the position of national leadership to which you aspire and which it is the presumed purpose of the Conservative Party to achieve.
"Believing that as I do, I clearly cannot honestly remain in the party. I do not intend to leave public life."
Mr Davies said he had found himself increasingly "naturally in agreement" with the Labour Party.
He praised Mr Brown as "a leader I have always greatly admired, who I believe is entirely straightforward, and who has a towering record, and a clear vision for the future of our country which I fully share".
Mr Brown said: "Quentin Davies is a senior parliamentarian and he commands respect on all sides for his expertise and his dedication to public service, and I welcome him to the new Labour Party."
'Slap in face'
Mr Cameron wrote that Mr Davies' decision did not come as a surprise to him.
"I am sorry that you feel unable to be part of today's Conservative Party, and join us in campaigning on what matters to people," Mr Cameron said.
He added: "Thank you for your support in the past. We will watch your future career with interest."
Lincolnshire County Council's Tory leader Martin Hill called the defection an "act of treachery and betrayal".
He said: "I think it's a slap in the face for all of those people who supported and went round for him.
"I feel very strongly, I don't approve of politicians who stand under one flag and then change to another flag for their own convenience. It is an act of treachery and betrayal, frankly."
David Davis, the shadow home secretary, said he was "extremely surprised" at the news.
Sir Nicholas Winterton, Conservative MP for Macclesfield, called for Mr Davies to resign his seat, adding: "All I say is 'good riddance'."
Tory peer Lord Tebbit said: "This defection will raise the average standard of members on the Conservative side and lower it on the Labour side."
Shadow industry secretary Alan Duncan said Mr Davies was "not socially liberal" and against "a changed Conservative Party".
He added: "And basically he's quite grand and old fashioned, and I'm surprised that he finds that the Labour Party will offer him a happy home for those attitudes."
But former Conservative MP Lord Temple Morris, who defected to Labour in 1998, said Mr Davies's decision showed "guts".
Last year, Mr Davies called Mr Cameron's decision to vote for an immediate inquiry into the Iraq war "absolutely crazy".
He also said the party risked looking like "dishonest double-glazing merchants" over plans to withdraw from the European People's Party group in the European Parliament.
The 63-year-old is a former diplomat and has been shadow Northern Ireland secretary and shadow defence secretary. He became an MP in 1987.