Ms Smith said: "We now face a threat level that is severe. It's not getting any less, it's actually growing.

"There are 2,000 individuals they are monitoring. There are 200 networks. There are 30 active plots.

"That has increased over the past two years. Since the beginning of 2007, 57 people have been convicted on terrorist plots.

"Nearly half of those pleaded guilty so this is not some figment of the imagination. It is a real risk and a real issue we need to respond to.

"We can't wait for an attack to succeed and then rush in new powers. We've got to stay ahead."

Under the new proposals the home secretary would be able to immediately extend the detention limit of a suspect to 42 days, as long as it was supported by a joint report by a chief constable and the director of public prosecutions.

The extension would then have to be approved by the Commons and the Lords within 30 days. But if either House voted against it, the power would end at midnight on the day of the debate.

The proposals are supported by some senior police officers - but could face a court challenge from the Equality and Human Rights Commission if passed.

Last November, Jonathan Evans, MI5's director general, spoke of 2,000 people posing a threat to the UK - the same number quoted by the home secretary.

However, he went on to say that the number had not peaked.

In November 2006, the then MI5 director general, Dame Eliza Manningham-Buller, again spoke of 30 different terror plots and said 1,600 individuals were being monitored.

Dame Pauline Neville-Jones, the shadow security minister and chair of the joint intelligence committee, told the BBC she did not agree with the government's approach.

"If there is an increase in plots it doesn't follow, I fear, that the right way to deal with that is actually then to hold suspects for an even longer time.

"What you need if you've got an increase in plots is the right quantum of resources for both the police and the intelligence services to track and disrupt the plots - and that's a question of bringing resources to bear."

Lib Dem home affairs spokesman Chris Huhne said the move to extend the time limit for holiding terror suspects was "ridiculous".

"We have a 92% conviction rate on terrorist offences. For heavens sake that's far higher than on ordinary criminal cases.

"So what on earth is the government on about in saying that we need to go further than any comparable country has anywhere else in the world?

"In Canada it's still one day, and we're talking about 42. In America it's only two days and we're talking about 42. In Australia it's 12 days."