The Home Secretary offered concessions on Tuesday to try to save Prime Minister Gordon Brown from a parliamentary defeat over plans to extend the time limit for holding terrorism suspects without charge.

Brown is pressing on with the plan even though he could face a rebellion by some members of his party and his popularity has sunk as economic problems mount.

Brown says allowing suspects to be detained for up to 42 days without charge is "the right thing to do to protect the security of all and the liberties of each", but critics say the measure treads on ancient liberties in an effort to look tough.

Hoping to win over sceptics, Home Secretary Jacqui Smith submitted amendments to the proposed law, under which the 42-day detention powers would only be used against a "grave exceptional" threat - envisaged as something on the scale of suicide bombings in London that killed 52 commuters in 2005.

She reduced the time before which parliament must approve any use of the powers to a week from a month, and said the police would only have access to the powers for a limited time.

"We have been very clear that extending the period of pre-charge detention would only ever happen if there were a very grave and exceptional terrorist threat," Smith told the BBC.

The opposition said the changes showed the government had no idea what it wanted.

Brown, whose party lost a safe parliamentary seat last month and was beaten into third place in local elections, has been facing the danger of a rebellion by up to 50 Labour members when parliament votes on the proposal next week.

His popularity has fallen as the global credit crunch has hit, with prices rising and growth slowing. Brown must call a parliamentary election by 2010 and hopes the economy will have had time to recover by then.

THE TIDE HAS TURNED?

A source close to Brown said the prime minister was confident the terrorism bill would pass and he wanted to stand firm on security, but his spokesman said the government still had work to do to get the votes it needed.

Home Office Minister Tony McNulty told BBC Radio: "Things do seem to be shifting back to the government's way."

The plans have been attacked by the government's former top lawyer, Peter Goldsmith, who described them as a "very serious incursion on our fundamental freedoms".

But the government was backed on Tuesday by former counter-terrorism chief Peter Clarke. He said plots uncovered since 2005 showed "the terrorist threat is growing in scale and complexity".

Four Islamist suicide bombers killed 52 London commuters in July 2005, a suspected plot to blow up transatlantic airliners flying from London was foiled in 2006 and last year car bomb strikes in the capital and Scotland were thwarted.

Civil rights groups say the 28 days that British police are already permitted to hold terrorism suspects is much longer than in countries such as the United States, Russia and Turkey. The proposals have also angered Islamic groups who say the powers could alienate many of Britain's 1.7 million Muslims.