The Conservative Party was tonight reeling from the extraordinary resignation of one of their most senior frontbenchers to fight an unprecedented "single-issue" by-election.

David Davis, a right-winger and one of the Tories' political heavyweights, stunned Westminster by announcing he was quitting as both an MP and Shadow Home Secretary to fight against the Government's "strangulation" of British freedoms.

Mr Davis, who lost the 2005 Tory leadership contest to David Cameron, has been the driving force behind Tory opposition to Gordon Brown's plans to extend detention without charge for terrorist suspects from 28 to 42 days. He is known as a staunch libertarian.

He reportedly threatened to resign unless Mr Cameron agreed to oppose the proposals, which were narrowly passed by Parliament last night. The Bill is expected to face a tough ride in the House of Lords before becoming law.

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'David Davis has shot us in the foot,' one senior Tory told me

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The Haltemprice and Howden MP, who will remain in the party and fight the election as the official Conservative Party candidate, is believed to have taken his closest Parliamentary colleagues by complete surprise with his statement.

Mr Cameron described the move as a "courageous" personal decision. Behind the scenes, however, it was greeted with huge concern, with some Tory MPs and grass-roots activists believing he had handed Gordon Brown an open goal after last night's narrow 42 days defeat.

There were fears that the by-election could turn into a farce with the Lib Dems deciding not to field a candidate in solidarity with him, while Labour - which labelled the move a publicity stunt - was also considering whether or not to compete.

There were Tory fears that the move would also draw out internal divisions within the party, with some senior Conservatives far weaker in their opposition to Gordon Brown's terror plans than Mr Davis and others having reservations about opposing it at all.

One MP said: "Here we are on the ropes and he goes and does this. Gordon must be in seventh heaven." Another asked: "Has he flipped?"

On the blog representing grassroots Tories, www.conservativehome.com, the majority of message boards disagreed with the move.

"Madness – thank the lord that he was never elected party leader! I hope the whip is removed - Let him stand as an independent candidate and let him find the funds to fight a by-election," one blogger wrote.

Another wrote: "Ambitious MP with large majority in publicity stunt shocker! Clever politics, but that's all it is." A further post said: "Admire DD but we really didn't need this."

A significant minority, however, supported him. One contributor wrote: "This will take the debate on freedom into every living-room in our country. Many people don't even know that children are fingerprinted or that someone can end up being arrested and have their DNA taken just for throwing out the wrong type of rubbish."

In his statement outside the Palace of Westminster, Mr Davis said he wanted to force a by-election to stop the "slow strangulation of fundamental British freedoms" by the Government, including the introduction of ID cards and the extension of detention without trial for terrorist suspects.

Claiming the government would soon seek to pass laws imprisoning suspects for "56 days, 70 days, then 90 days," he said: "In truth, perhaps 42 days is the one most salient example of the insidious, surreptitious and relentless erosion of fundamental British freedom."

Listing a string of what he said were government infringements on personal liberty - including a DNA database "bigger than any dictatorship", excessive CCTV cameras and an assault on jury trials - he said: "This cannot go on, it must be stopped, and for that reason today I feel it is incumbent on me to take a stand.

"I will argue in this by-election against the slow strangulation of fundamental British freedoms by this Government.

"Now, that may mean I have made my last speech to the House. It's possible. And of course that would be a cause of great regret to me.

"But at least my electorate and the nation, as a whole, would have had the opportunity to debate and consider one of the most fundamental issues of our day.

"And if they do send me back here, it will be with a single, simple message - that the monstrosity of a law that we passed yesterday will not stand."

After the statement Mr Cameron appointed Dominic Grieve, the Shadow Attorney-General, in as Shadow Home Secretary place and made it clear the appointment was permanent.

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