London - Prime Minister Gordon Brown Wednesday strongly defended controversial plans to detain terrorism suspects in Britain for up to six weeks without charge.

Speaking in parliament just hours before a crucial vote on the issue, Brown said he would "fail in his duty" if he did not back proposals to extend the period for which suspects can be held from the current 28 days to 42 days.

"It is a matter of necessity because of what the police and the security services have told us," Brown said to jeers from the Opposition and from his own Labour benches.

With a number of Labour "rebels" expected to oppose the measure, or to abstain, the vote later Wednesday could turn into Brown's first major parliamentary defeat since he came to power a year ago.

"The prime minister's authority is under the spotlight," said one analyst.

While Brown argued that an extension of detention was needed to tackle increasingly "complex and sophisticated" terror plots, critics have warned against the erosion of civil liberties.

"Terrorists want to destroy our freedom. When we trash our liberties, we do the terrorists' work for them," said Conservative leader David Cameron.

Former Labour government minister Frank Dobson, who is leading the rejection camp, said Britain's tradition of civil liberties was under threat.

"I believe this is quite a fundamental issue: we have had the right not to be locked up for a long time without charge since the year 1215 - this is one of the very few things that is actually in Magna Carta and we need to sustain it," said Dobson.

A revolt by 34 Labour members of parliament (MPs) would suffice to defeat the government. The Labour Party has a majority of 66 in the Lower House of Parliament.

In a sign of how tight the vote is, the government took the unusual step of urging Foreign Secretary David Miliband to cut short a visit to Israel Tuesday.

The plans have been condemned by human rights groups as a violation of basic liberties.

However, a number of senior police chiefs, the head of the security services, and the chief prosecution agency have also said that they see no requirement for an extension of pre-trial detention.

Application of the 42-day-rule would in each case be subject to a number of judicial and parliamentary safeguards.

It was reported Wednesday that in a last-minute concessions to rebels, the government was considering to pay compensation of up to 3,000 pounds (6,000 dollars) a day for every day suspects were held but subsequently released without charge.

In 2005, former prime minister Tony Blair suffered his first major parliamentary defeat when he tried to push though an extension of detention without trial to 90 days. (dpa)