Police in London have been put on their highest state of alert after warnings were received that an al-Qa'ida suicide attack may be imminent.
Frontline officers in the capital are being given fresh instruction on spotting and tackling suicide bombers. Similar guidance is to be issued to officers in other large urban forces next month.
The moves come after the FBI uncovered and passed on new intelligence that the threat was growing of an al-Qa'ida outrage in the capital.
Senior officers were briefed on the threat a fortnight ago in a presentation at Scotland Yard's firearms unit.
They are understood to have been told that al-Qa'ida's presence in Britain was substantial and capable of staging an attack.
The guidance for officers on the beat will be contained in a counter-terrorist manual compiled by Chief Inspector Andy Latto, Scotland Yard's senior firearms instructor.
"Suicide bombers are getting better at killing people," he told The Sunday Telegraph. "The increase in deaths is far superior to the increase in incidents.
"We are currently briefing chief officers across the country and the information will cascade down to officers."
A Metropolitan Police spokeswoman refused to be drawn yesterday on details of security measures. "We have a record that is second to none in the world on counter- terrorism and we're confident this will continue," she said.
Patrick Mercer, a Conservative home affairs spokesman, accused the Government of not taking the threat seriously enough. "It is clear attacks are being planned in this country," he said. "We mustn't let our guard drop.
"Several of my visits over the past few weeks have shown me that little has been done in the NHS, the fire service, the police and local authorities to prepare for such attacks."
Undercover police teams tracking suspected Islamic terrorists in London were armed for the first time earlier this year and instructed to shoot to kill if they believed a suspect was about to detonate a bomb strapped to his body or in a vehicle.
The Metropolitan Police has already been placed on maximum alert twice this year - in January, when the deadly poison ricin was discovered at a flat in Wood Green, north London, and in February, when troops and tanks were dispatched to Heathrow airport.
Security has also been stepped up at potential targets such as the Houses of Parliament, now protected against car bomb attacks by a low concrete wall.