Jacqui Smith, who faces a serious Labour rebellion over plans to extend the time suspects can be held without charge to 42 days, said it was vital to "stay ahead".

She must calm intense anger within the party if the Government is not to suffer a repeat of the damaging 2005 defeat of proposals to extend the limit to 90 days.

Former cabinet minister Frank Dobson warned last week that opposition was stronger now because no evidence had been produced of a need to go beyond the present 28-day limit.

The Director of Public Prosecutions Sir Ken Macdonald is among a number of senior figures - including Security Minister Lord West of Spithead and ex-Attorney General Lord Goldsmith - to have voiced misgivings about the plan.

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

She went on: "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."

She went on: "When we extended the period from 14 to 28 days people said 'there's no need for this, it's not necessary. You'll never need to use it'.

"Well, we did. The number of international investigations are greater than before. Each time a plot is uncovered the terrorists learn and they develop. That is why there is a massive increase in the way they are using technology and encrypting evidence. It takes time to get the evidence you need to charge somebody."