LONDON — Prime Minister Gordon Brown warned Monday that he won't back down over contentious plans to toughen Britain's anti-terrorism laws.

Brown wants to give police up to 42 days to detain suspects before charges are filed — extending the current limit of 28 days — to give investigators time to unravel increasingly complex terrorist plots.

Civil liberties campaigners, a number of lawmakers and Britain's chief prosecutor oppose the plans as unnecessary and draconian.

Opponents claim that holding suspects for long periods without filing charges would likely increase public distrust of the police and government, particularly among Britain's Muslim communities.

Lawmakers are to vote on the plans next week, and some legislators from Brown's own Labour party are expected to oppose his proposed new laws.

"I will stick to the principles I have set out and do the right thing: protecting the security of all and the liberties of each; and safeguarding the British people by a careful and proportionate strengthening of powers in response to the radically new terrorist threats we now face," Brown wrote in an opinion piece in the Times.

Tony Blair, Brown's predecessor, suffered a humiliating first parliamentary defeat in 2005 when lawmakers voted against his plan to extend the maximum detention to 90 days. Legislators instead approved a 28-day limit.

Under the proposed new laws, the detention of a suspect for more than 28 days would need approval from Britain's chief prosecutor, by Parliament and — every seven days — by a judge.

Next week's vote comes as Brown is reeling from damaging defeats in May local elections and a special election in northern England that saw the opposition Conservatives capture a longtime Labour stronghold.

John McDonnell, a Labour lawmaker who said he will vote against the plan, insisted rebels aren't seeking to undermine Brown.

"Voting against this is not a challenge to the Prime Minister's leadership," he said. "It's a fundamental issue about human rights. Let's keep it away from party politics."