Britain warned of a growing terrorist risk from chemical, biological and radiological weapons and pinpointed al Qaeda in Pakistan as a key threat as it revamped its national security strategy.
Prime Minister Gordon Brown's government also said its updated counter-terrorism policy would focus efforts on trying to prevent young Muslims from being radicalised by violent Islamists, and involve ordinary Britons in tackling the threat.
"We need to tackle the causes of terrorism. We need to get in early to prevent people actually supporting violent extremism or supporting terrorism," Home Secretary (interior minister) Jacqui Smith told BBC radio.
Britain has been a target for Islamist militants since it joined the United States in invading Afghanistan and Iraq after the September 11, 2001 attacks. In 2005, suicide bombings in London killed 52 people.
Between 2001 and 2008, more than 200 people were convicted of terrorism related offences, and the national threat level remains at its second highest level of "severe", meaning an attack is highly likely.
The security strategy updates a 2003 policy based around principles of prevent, pursue, protect and prepare.
It says Britain probably has the most numerous and capable al Qaeda cells in Europe, and warns there is a growing possibility that militants' hopes of using chemical, biological and even nuclear weapons could become a reality.
"Changing technology and the theft and smuggling of chemical, biological, radiological, nuclear and explosive (CBRNE) materials make this aspiration more realistic than it may have been in the recent past," the document says.
While the strategy highlights threats from a number of sources, ministers and officials say militants in Pakistan pose the greatest concern, reflecting the views of US President Barack Obama's administration.
British security services say there have been Pakistani links to almost all of the dozen major terrorism plots foiled since 2001 as well as the 2005 bombings, while Brown warned on Monday of an "al Qaeda core" in northern Pakistan.
Sky News, citing unnamed sources, said Pakistan's intelligence service had identified more than 20 Britons who had been trained by militants in Pakistan and had returned to Britain, where they posed a security threat.
Although the government will spend about 3.5 billion pounds ($5.10 billion) a year on counter-terrorism by 2011, Smith said the country could not depend solely on the security services.
Some 60,000 ordinary Britons, from security guards to store managers, are being trained to deal with incidents and to spot potential suspects in crowded places such as shopping centres.
Smith said there would also be a focus on encouraging communities to "challenge" those with extremist views but whose message did not go as far as breaking laws on inciting violence.
"We should argue back, we should make clear those things are unacceptable," she said.
The Muslim Council of Britain said 200 religious and civic leaders had concluded the new strategy was counterproductive.
The government "may be in danger of adopting misguided notions of extremism as dictated by xenophobic commentators," said the MCB, Britain's largest Islamic umbrella group.