The British government's widely-opposed plans to hold suspected extremists for up to six weeks took a new twist Thursday, as a senior opposition figure resigned to fight a by-election on the issue.

The unprecedented decision by Conservative Party home affairs spokesman David Davis gives Prime Minister Gordon Brown another electoral headache, after crushing defeats at local polls and a by-election last month.

Brown's Labour Party already faces certain defeat in the Tory stronghold of Henley, near Oxford, southern England, later this month, where the seat is vacant after Boris Johnson became the new London mayor.

Another defeat -- likely as Davis had a comfortable 5,116 majority at the last general election in June 2005 -- and further public scrutiny of the controversial proposals will increase the pressure on Brown.

The smaller opposition Liberal Democrats, Davis's most likely challengers, also said they would not put up a candidate against him.

Brown scraped victory by nine votes in parliament Wednesday over the plans to increase pre-charge detention limits for suspected extremists from 28 days to 42, but only with the help of the Democratic Unionist Party (DUP) of Northern Ireland.

Thirty-six Labour members of parliament rebelled amid a clamour of opposition from civil rights groups to senior prosecutors and claims that Brown had consigned ancient ideas of liberty and democracy to history.

But at a news conference earlier Thursday, Brown repeatedly denied any backdoor deal with the nine Northern Irish members of Parliament, saying: "I don't do deals on terrorism."

He also said opposition parties would regret not backing the proposal.

But his protests are likely to be drowned out by Davis and his vow to fight the "monstrosity of a law that was passed yesterday" and what he said was the increasingly intrusive power of the state over the individual.

Davis, the Tory's home affairs pointman since 2003, told reporters outside parliament that Brown, who argues an extension is needed because of the increasingly complex nature of alleged plots, had not made the case adequately.

And Davis said the government would be tempted to use rare political powers to force through the Counter-Terrorism Bill, even if it is rejected in the upper chamber, as is expected.

"Next we will see 56 days, 72 days, then 90 days. But in truth, 42 days is just one example of the insidious and relentless undermining of fundamental British freedoms," he said.

Davis said he would fight for re-election in his Haltemprice and Howden constituency in northern England on the government's erosion of civil rights.

In particular he pointed to the proposed biometric identity card scheme, the proliferation of CCTV, the massive expansion of the DNA database, government plans to reduce trial by jury and restrictions on peaceful protest.

"I will argue in this by-election against the slow strangulation of fundamental British freedoms by this government," he said.

Tory leader David Cameron described the decision by his former opponent for the party's top job as "courageous" and pledged his support. Shadow attorney general Dominic Grieve was appointed in Davis's place.

Senior Labour figures dismissed Davis's surprise announcement as a "stunt" while chatter among Britain's robust political blogging community questioned whether a row between Cameron and Davis prompted his decision.

Davis said a Conservative government would repeal the 42-day rule if it gets on the statute books, but Cameron has made no such commitment.