If David Davis's aim was to take the shine off the restrained Labour celebrations over the 42-day terror vote, then he has probably succeeded. But as news of his shock resignation spread around Westminster this lunchtime it was Labour MPs, and not their Tory counterparts, who had a spring in their step. One told me: "Here we are on the ropes and he goes and does this. Gordon must be in seventh heaven."

Nothing is predictable in politics and the Shadow Home Secretary's decision was one of the most unpredictable ever. His friends called it brave. Other Conservative MPs called it mad. "Has he flipped?" was a question doing the rounds in all the corridors. It's quite obvious that his decision did not please David Cameron, who was told about it on the telephone some time after last night's vote. He tried to talk Mr Davis out of it, but his mind was made up. To him the loss of ancient freedoms has become such an issue that it transcends all others and he is going to fight a by-election on it.

Who – if anyone – will fight against him is not yet clear. He secured a promise from Nick Clegg, the Liberal Democrat leader, that they would not put up a candidate against him because they take the same line on 42-day detention. Labour seems highly unlikely to fight, but will keep the Tories guessing until Mr Davis has formally resigned.

Mr Cameron meanwhile has appointed a new Shadow Home Secretary in Dominic Grieve and made plain that he is not a stopgap. Mr Davis will not walk back into the Shadow Cabinet after his certain re-election next month.

Mr Davis's friends say that he is doing what he is doing to give fresh justification to the Lords if they decide to block the 42 day plan. Peers are widely expected to defeat the measure, but will they provoke a constitutional confrontation by refusing repeatedly to allow the Bill through after it has ping-ponged between both Houses? That would mean the Government had to force the Bill through using the Parliament Act next year. "If David comes back with a big majority having stood solely against 42 days they (the Lords) can feel they have complete democratic legitimacy," a friend of Mr Davis said.

The Tory conspiracy theorists see a more sinister motive, wondering whether Mr Davis, who has twice been thwarted in bids for the leadership, is positioning himself for a third try if anything goes wrong for Mr Cameron. Is he doing a Heseltine, some MPs are asking, referring to another age when Michael Heseltine left the Cabinet over Westland to leave himself ready for a challenge for the leadership.

But Tory MPs are doubtful over what the impact may be of a by-election where there is only one candidate from the main parties. Mr Davis will get some publicity in the early days of the campaign, and is dominating the airwaves today. But the campaign may rather quickly become boring. Perhaps an independent candidate will spring up from somewhere to back the 42 day power. But even though the polls suggest the public favours 42 days, Mr Davis, as the sitting MP, would be expected to get back with a massive majority.

Today is one of Mr Brown's best days for some time and one of the least comfortable of Mr Cameron's leadership . "Davis has shot us in the foot," one senior Tory told me.

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