London - The British government warned Tuesday that a terrorist attack on the country was "highly likely" and could "happen without warning, at any time."

Home Secretary Jacqui Smith sounded the warning as she presented a terror strategy report to parliament which said that the main threat came from a terrorist plot involving a so-called dirty bomb.

While not explicitly linking her comments to the G20 finance summit in London next week, Smith said threat levels in the British capital were at "severe," one category below the "critical" level which means an attack is imminent.

Smith said that over a dozen attempted terrorist plots had been disrupted by the police in recent months and it was her aim to reassure the public that everything was being done to pursue terrorists and prevent violent extremism.

"We know that the threat is severe. We know that an attack is highly likely and could happen, without warning, at any time. And we know that this new form of terrorism is different in scale and nature from the terrorist threats we have had to deal with in recent decades," she told parliament.

The greatest security threat came from "al-Qaeda and related groups and individuals" mainly on the Pakistan/Afghanistan border, said Smith.

London's public transport network became the target of a double suicide attack on buses and the Undergound (Tube) system on July 7, 2005, while the leaders of the G8 industrial nations were holding a summit meeting in Gleneagles, Scotland.

Fifty-two people died in the bombings and more than 700 were injured.

A government assessment of the terrorist threat showed that while terrorism networks such as al-Qaeda were "likely to fragment" and might not survive in their current form, their ideology would live on, Smith quoted fro the report Tuesday.

The "likely future direction" of the threat would involve the use of a so-called dirty bomb, the report said. "Contemporary terrorist organizations aspire to use chemical, biological, radiological and even nuclear weapons."

In these circumstances, the anti-terrorism fight could not be left to the police and governments alone, said Smith, unveiling plans for a wider involvement of the public in combating the danger, Smith warned Tuesday.

She said hotel managers and security staff would be among the approximately 60,000 workers that would be trained "in vigilance for terrorist activity and what to do in an attack."

Smith said the strategy would also tackle "anti-democratic extremist voices" in the community while supporting moderate groups which backed democracy.

People who may not have broken the law but nevertheless "act in a way that undermines our beliefs in this country in democracy, human rights, tolerance and free speech" should be challenged.

"We should argue back and make clear that these things are unacceptable," said the Home Secretary. (dpa)