Leading article: A twist of the knife

Quentin Davies was hardly the most prominent Opposition MP. Nor, in conventional ideological terms, was he the most consistent. As a Thatcherite and ardent pro-European, he arguably combined the two warring strands of today's Conservatism. It is also true that a cloud of doubt, even treachery, inevitably descends on any sitting MP who signs up to another party and declines to stand down before the next election.

Even if we apply all these caveats, however, the defection of Mr Davies to Labour cannot but be a damaging blow to the Conservative leader, David Cameron - and a corresponding boost to Labour. The timing, on the eve of Tony Blair's departure from No 10, is exquisite. What better welcome present could the new Prime Minister receive than another MP to vote with his Government?

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One of Mr Davies's complaints - that Mr Cameron was wrong to be pulling Tory MEPs out of the centre-right bloc in the European Parliament - is old hat. Mr Davies could have made it at any time since Mr Cameron became leader. If - as a Kenneth Clarke-style pro-European - it is the whole spirit of the leader's Europe policy he objects to, he could have found a reason to defect under any of the post-Major leaders.

While Europe featured prominently in the resignation letter Mr Davies made public, however, it came across as an adjunct, rather than the central point of his distemper. The real venom - in paragraphs which could have come straight from Labour Party HQ - concerned David Cameron's leadership of the party.

On the charge sheet were vacuity, vacillation and making policy on the hoof. Under Mr Cameron's leadership, he says: "the party appears to me to have ceased collectively to believe in anything or to stand for anything. A sense of mission has been replaced by a PR agenda." Foreign policy was "a shambles".

And while Mr Davies denied any animus against Mr Cameron, it is on character and leadership that he twists the knife. "Although you have many positive qualities," he writes, "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..."

Such accusations are damaging in themselves, but doubly so for Mr Cameron because they echo complaints that can be heard in Tory party circles and, increasingly, from potential voters. Coming at a time when Mr Cameron is struggling to overcome his first real contretemps with his party and Labour voters are anticipating an end to Blairite "spin", Mr Davies's defection, and his stated reasons, offer Mr Brown the sort of morale boost he can have imagined only in his dreams.