Members of the public will be warned if there is a specific terrorist threat against a known target, Prime Minister Tony Blair has pledged.

But the government had to be "very wary" of acting on general information which could panic people and end up "doing the terrorists' job for them", the prime minister said.

If there is a specific threat against a specific target, we of course will warn people Tony Blair

Mr Blair's promise followed reports that police and MI5 had thwarted a poison gas attack on the London Underground.

Three North African men have been charged under anti-terror laws with the possession of articles for the preparation, instigation and commission of terrorism acts - but there was no allegation of a plan to target the Tube.

Rabah Chekat-Bais, 21, Rabah Kadris, in his mid-30s, and Karim Kadouri, 33, all of no fixed abode, were remanded in custody by Bow Street magistrates in central London on Monday.

'Wary'

In an interview with ITV1's Tonight with Trevor McDonald, Mr Blair said: "The difficult dilemma we have here is this - constantly at the moment, because of the terrorist threat, we have pieces of information, pieces of intelligence, coming across our desks and we have to sift that and weigh it and judge it."

He insisted: "If there is a specific threat against a specific target, we of course will warn people.

Blair: Keeping a constant watch over intelligence on terrorism

"But we have got to be very wary of acting on general information, of issuing warnings whey they are not really justified according to evidence.

"The risk then is we unnecessarily alarm people, we end up doing the terrorists' job for them, and for the public, they will get confused if there are different pieces of information coming at them the whole time.

"Of course I keep a constant watch and am constantly trying to assess and re-evaluate if we are making the right moves."

Mr Blair has already briefed Tory leader Iain Duncan Smith in Downing Street on the terror threat to the UK, in the wake of fresh warnings about possible attacks by al-Qaeda.

The Conservatives have also pressed Home Secretary David Blunkett to give a full statement to the House of Commons, but that has been turned down for the time being, a spokesman said.

Earlier on Monday, Foreign Secretary Jack Straw said Britons should be on full alert in readiness for an attack by al-Qaeda.

'No cyanide'

He was speaking as the three men of North African origin, were charged under Section 57 of the Terrorism Act 2000.

They appeared in court last week and were remanded in custody to appear before magistrates on Monday, when they were again remanded in custody until 16 December.

Police said no cyanide or noxious gas had been found on the men when they were arrested on 9 November.

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

Not a 'Bond movie'

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."

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

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

But Mike Brown, London Underground's customer services director, said it was ready to run a poster campaign to advise customers about what to do in a possible gas attack on the Tube.

'When, not if'

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