The government's controversial bid to extend the pre-charge detention period for terrorism suspects beyond 28 days will form the centre of new security proposals being unveiled on Thursday.

In the counter-terrorism bill - a key plank of Prime Minister Gordon Brown's legislative agenda for this year - ministers will call for police powers to hold terrorism suspects without charge for up to 42 days.

"We are monitoring 2,000 individuals working in 200 networks with 30 active plots. This is a threat that is not going away. In fact, if anything, over the last few years it has grown in significance," Home Secretary Jacqui Smith told Sky News.

With reports of a potentially major rebellion from within the ruling Labour Party, the government is expected to offer assurances that civil liberties will be protected through a system of judicial and parliamentary oversight.

"It doesn't come into operation now. It would only ever come into operation if the circumstances required it, if very high legal thresholds were covered, with the approval of parliament," Smith said.

"We don't need it now. But there is a consensus we may need it in the future. The responsible thing to do ... is to legislate now but for something you would only use if the circumstances arose," she added.

The bill aims to give police and the security services the necessary tools to deal with a rising threat from home-grown and foreign terrorists.

But ministers have some hard bargaining to do to persuade opposition parties to support their plans to avoid a damaging defeat in parliament for Brown.

"I think we have won the debate on extending detention, on constantly ratcheting up this figure with no real principle and no new evidence of the need to go beyond 28 days which is already the longest period in the Western democratic world," said Shami Chakrabarti of human rights group Liberty.

A proposal to increase the maximum period for which suspects can be detained without charge to 90 days led to Tony Blair's first defeat in the House of Commons in November 2005 as many Labour MPs rebelled.

MPs settled on the 28-day limit.

The government argues that a 28-day period is insufficient given the complex nature of terrorist plots.

Smith, under fire for saying that she did not feel safe walking the streets of London, has said 42 days would be the absolute maximum.

Civil rights groups like Liberty and Amnesty International argue that pre-charge detention for 42 days contravenes people's basic human rights.