The arrest of three men in London has raised concerns of a heightened terrorist threat from al-Qaida operatives in Britain.

Scotland Yard officials say the three men of North African origin are charged under Britain's Terrorism Act with plotting to commit a terrorist attack.

That is as much detail as they are willing to release. But the Sunday Times newspaper of London reports that the men were planning to release poisonous cyanide gas on the London subway system.

Nick Rufford, an assistant editor for the Sunday Times, said the three had been under surveillance for months. Then last week, he said, the signal was given to apprehend them. "We understand that it was authorized at the very senior level by the prime minister at a cabinet meeting and the security services were instructed essentially to wrap up their operation and the police moved in and made the arrests," he said.

Britain's Deputy Prime Minister John Prescott, speaking on British television, has attempted to clarify the specifics of the case and to calm public fears in Britain. "The security forces in this sense have arrested and the police have arrested three people who were in their view about to commit acts. But they did not have any evidence of a bomb or any evidence of a gas. That is left for the fertile imagination of the press. What we have to do is get on with catching them, as we have done in this case, and trying to maintain the highest alert. And we advise the public to do exactly the same," he said.

Meanwhile, Britain's foreign secretary has warned Britons of another potential danger: travel to Yemen. He said British citizens already there should consider leaving over fears of extremist attacks based upon new intelligence information.

The warning follows the release last week of an audio tape believed to be Osama bin Laden in which he calls on his followers to launch fresh attacks against Western interests.