By Nick Assinder
Political correspondent, BBC News website
The joke already doing the rounds in Westminster concerning Quentin Davies' defection to Labour is that he is a bit too left-wing for Gordon Brown.
Mr Davies has been critical of recent Tory policies
Mr Davies is well known for his leftish views. He is what was once called a "wet" in Margaret Thatcher's days, particularly over Europe.
He was a great supporter of pro-European Tory leadership candidate Kenneth Clarke and has regularly criticised aspects of his party's policies.
He recently disagreed with Tory ideas for a flat tax rate and that led to a put-down from shadow chancellor George Osborne which summed up Mr Davies' relationship with the party.
"Well, I'm afraid to say I actually disagree with almost everything Quentin Davies has ever said. I often find myself on the wrong side of the argument with him, even though we're both Conservative MPs," said Mr Osborne.
He has long been considered by some in Westminster as the man most likely to defect to either Labour or the Liberal Democrats.
None the less, a defection is never something a political party welcomes.
It has clearly been well planned to come just as Gordon Brown is taking over as prime minister and looking for ways of hitting the Tories.
The fact Mr Davies has gone claiming Mr Cameron appears not to believe in anything will maximise any embarrassment felt by the Tories.
He has hit on just the criticisms that have been aimed at Mr Cameron, usually from the more traditionalist wing of the party.
Some in the party may actually claim they are pleased to have a potential irritant out of their ranks.
Martin Hill, the Tory leader of Lincolnshire County Council, has not minced his words, saying: "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"
Mr Davies is seen as a straightforward and decent politician, although it seems his frontbench career is now over.
But Mr Brown will see it as a feather in his cap to have brought over a defector even before he has officially become prime minister.
He has regularly spoken of his wish to reach out beyond tribal politics to all the talents, and this will help give weight to that ambition.
Whether he will give Mr Davies any sort of official role is not known, although the Labour Party is not normally over-friendly to former Tories, no matter how repentant they are.
What the defection amounts to is another blow to David Cameron, although a small one and a nice bit of early PR for the Brown premiership.