British police and security agencies are currently monitoring 30 terrorism plots, Home Secretary Jacqui Smith said in extracts of a newspaper interview released on Saturday.
"We now face a threat level that is severe. It's not getting any less, it's actually growing," she said in an interview to be published in Sunday's News of the World.
"We task the police and the security agencies with protecting us ... There are 22,000 individuals they are monitoring. There are 200 networks. There are 30 active plots," she said.
Prime Minister Gordon Brown's Labour government is seeking to extend pre-charge detention of terrorism suspects to 42 days from the current 28-day limit.
But Smith faces a tough task steering the controversial provisions through parliament.
The Conservative and Liberal Democrat parties have both said they will vote against extended detention. Labour backbenchers are also threatening to rebel and vote down the clause in the Counter-Terrorism Bill.
"We can't wait for an attack to succeed and then rush in new powers," Smith said. "We've got to stay ahead.
"Because we now understand the scale of what is being plotted, the police have to step in earlier, which means they need more time to put evidence together."
Britain has seen a marked increase in militant Islamist plots since it joined the United States in invading Iraq in 2003.
In 2005 four British suicide bombers killed 52 people in London. Other attempts have been thwarted by police or failed when devices did not detonate.
"Since the beginning of 2007, 57 people have been convicted on terrorist plots," said Smith.
"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."
