The UK could be at risk of an attack on the scale of the Bali nightclub bombing, the government's anti-terror watchdog has warned.
Twenty four Britons died in the Bali bombing
"Ordinary people" rather than politicians are most at risk, especially at venues where large groups gather, he said.
The stark message was delivered at a House of Commons home affairs select committee by Lord Carlile of Berriew QC.
Some 202 people died when a bomb ripped through a popular nightclub on the Indonesian island of Bali last October.
Despite saying he did not want to promote "doom and gloom", Lord Carlile urged the whole country to be on continued alert for suspicious activity in the UK.
Lord Carlile was appointed on 11 September 2001 - coincidentally - to review the functioning of the Government's Terrorism Act and, later, the
Anti-Terrorism, Crime and Security Act.
Addressing the committee of MPs on Tuesday he said people should be aware of the "the enemy in our midst".
Ordinary citizens rather than politicians and
royalty were at greatest risk, he said.
"The people in greatest danger are our friends, sons and daughters who gather in places of large public aggregation and we have to be very vigilant to protect them," he said.
"What happened in Bali is a lesson."
Twenty-four Britons were among 202 people who died in the Bali bombing, when Islamic terrorists set off a car bomb outside a nightclub on the island.
A suicide bomber detonated a second device in a nearby bar moments later.
Troops and tanks boosted security at Heathrow Airport
Lord Carlile also told MPs that several aspects of security at ports and
airports needed tightening.
Some airports need better checks on airline crew, he said.
He said it seemed, after he made inquiries at Heathrow, that a "rogue state or rogue organisation" could infiltrate an airline and possibly a crew.
"That causes me concern. And my real concern is the big, international airports."
But he said small ports and airports, and even coves on the British coastline, still needed to tighten security.
"We have to remember that lethal material could be brought into this country on a small yacht into a small harbour anywhere around the coastline," he said.
"It's impossible to provide a fool-proof system. "But it should be a sieve with a finer mesh than we have got at the moment."
He stress that the logging of passengers details recorded by airline and ferry operators should show consistency, recording names, addresses, dates of birth and passport numbers as a minimum.