Mr Straw was speaking as three men made a second court appearance in London after being charged under the Prevention of Terrorism Act.

He said the threat to the UK was on a par with that offered by the Provisional IRA at the height of their bombing campaigns.

People have to be on alert in the way they were during an IRA bombing campaign. Jack Straw

Mr Straw, who has ordered the closure of the British Embassy in Yemen following safety concerns, told BBC Radio 4's Today programme: "People have to be on alert in the way they were during an IRA bombing campaign."

He conceded the nature of the threat was different but disagreed that the intelligence services had known more about the IRA's activities than they did about those of al-Qaeda.

"The IRA were able to carry out some terrible atrocities right up until they signed the Good Friday Agreement," he said.

'Not a Bond movie'

Earlier Mr Straw said the flows of intelligence were difficult to interpret and it was not as simple as a "Bond movie".

But he said: "Where we believe there is good information about a serious and credible threat to a specific location then of course public safety is paramount."

The reality is no one can protect absolutely against ruthless terrorists who are prepared to lose their own lives at the same time. Lord Harris

Mr Straw's comments came amid suggestions that a poison gas attack was being planned on the London Underground.

But the foreign secretary denied there was a "credible" threat to the Tube.

The prime minister's official spokesman said later that if the government believed it was necessary to close down parts of Britain's infrastructure or to issue a specific warning, it would do so without hesitation because public safety was paramount.

He added the government did not plan to give a running commentary on police or intelligence operations.

Lord Harris, chair of the Metropolitan Police Authority, said the head of specialist police operations in London thought it was a case of when, not if, an attack happened in the capital.

'The unthinkable'

He told Today: "Following 11 September government minister) Nick Raynsford convened what is called the London resilience committee which looked at all the existing plans and upgraded them on the basis of what if the unthinkable happened.

"No one had envisaged terrorism on the sort of scale that we saw in New York and Washington that day.

"What if various things happened including attacks on the London Underground?

"Do the emergency services have the plan in place to handle specific events?

"All of those plans were upgraded. A lot of work was done on working out how agencies would cope under specific eventualities."

Lord Harris said: "We are much better prepared than we would have been 18 months ago.

"But of course the reality is no one can protect absolutely against ruthless terrorists who are prepared to lose their own lives at the same time."