Decision has hurt his leader the most

A messenger handed him a note revealing that the shadow home secretary was quitting. Around a table that had been resounding with recriminations over Labour's disastrous recent election defeats, phones began beeping similar messages.

Mr Brown grasped instantly that this extraordinary event might give more trouble to Tory leader David Cameron than to himself - and could even halt the steady Conservative poll gains in their tracks. Later, after watching in disbelief a video of Mr Davis's quixotic resignation speech, he reeled out instant plans to paint the Conservatives as soft on crime and terror.

"So, do the Tories say we should not use DNA to solve crimes? Or CCTV? If it wasn't for CCTV we would never have identified the 21 July bombers!" he said.

The paradox of Mr Davis's brave decision is that the chief victim of his actions is his own leader. That may not be accidental, because Mr Davis had to fight to persuade Mr Cameron to oppose 42 days and feared his party could backslide.

Friends say he is aware that he may finish his career on the backbenches and accepts the fact. Others fear he will be a menacing presence behind Mr Cameron's back, playing the part of the party's conscience on civil liberties and ancient freedoms. One Tory ex-minister simply said: "Thank goodness he did it now and not later. It just shows it is in his blood and he would as easily have walked out when we were in government."

Labour will not stand against him, seeing no point in losing a deposit at Haltemprice and Howden when there is more profit in letting Conservative strains ferment on their own.

Mr Cameron chose Dominic Grieve as the new shadow home secretary, partly to show that policy was not being changed, but he is now saddled with a less charismatic figure in one of the key campaigning posts.

And while the Westminster village may scorn Mr Davis, many grassroots Tories are full of admiration. The website ConservativeHome - which backed Labour on 42 days - said it had been swamped with support for him.

While Mr Davis may be denied his old job when this is over, his leader will not dare ignore him.