Islamist terrorists have stepped up their efforts to develop a "dirty" bomb for use against Western targets, senior security sources have told the Daily Telegraph.
They are exploiting political chaos in Pakistan in an attempt to acquire nuclear material for a "spectacular" attack.
At least one plot has been uncovered involving Pakistan-based terrorists planning a dirty strike against a major European target.
Al Qaeda, whose terrorist infrastructure is based in Waziristan province, northwest Pakistan, is known to be trying to acquire nuclear technology to attack the West. Other Islamist groups, such as the newly-formed Pakistani Taliban, have shown interest in making weapons with nuclear capability, Western security officials said.
Security chiefs fear instability in Pakistan will make it easier for them to do so.
Pakistan is the only Muslim country with a nuclear arsenal, which was developed in the 1990s by the rogue scientist Dr. Abdul Qadir Khan.
He was placed under house arrest after being accused of selling the blueprint for Pakistan's atom bomb to states such as Libya, North Korea, and Iran. But the restrictions on Dr. Khan's detention have been eased since President Musharraf was forced from power.
Pakistan's nuclear arsenal has stringent security safeguards, put in place with the help of the American military when Mr. Musharraf was in office. But there is mounting concern that Islamic terrorist groups will gain access to the expertise.
"Islamist militant groups want to carry out terror attacks on a massive scale, and there is no better way for them to achieve that objective than to develop some form of primitive nuclear device," a senior American security official said.
In a dirty bomb, conventional explosives are fitted with radioactive material.
Security experts believe the detonation of such a device in a city such as London would provoke widespread panic and chaos, even though the area of contamination would be relatively small.
Western security officials said they had uncovered evidence that a Pakistani based group was planning to attack a European target with such a device, although details have not been made public.