The government was on Monday accused of leaving the public in the dark over the possible threat of a terrorist attack using chemical or biological weapons.

Tony Blair's warning last week that such weapons could be used has alarmed some Britons, and sales of gas masks and protective chemical suits have soared.

"We've sold a lot more masks and suits than normal over the past week," said Howard Davis, manager of the Army & Navy Surplus Store in Victoria, London. "People are frightened."

A spokesman for the Cabinet Office said the government was not going to tell "people to go out and buy gas masks because we don't want to scare them unnecessarily".

"If there was a need for members of the public to take particular action, we'd communicate it in the appropriate way," he said.

But John Eldridge, editor of Jane's Nuclear, Biological and Chemical Defence, criticised the lack of clarity regarding which agencies were responsible for ensuring public safety in the event of an attack.

"My first worry would be that it's not quite clear who is in charge," he said. "There are so many authorities involved - the fire service, local authorities, the ambulance service - but no cadre that is beholden to what happens.

"Western governments - barring Israel and Denmark - have no real infrastructure for dealing with these kind of events," he said.

"What's worrying people at the moment is that the government is not saying: 'There's no chance of this happening.'"

The lack of government advice on the best course of action during a chemical or biological attack is in contrast to the volume of information made publicly available during the cold war on how to survive a nuclear assault.

Advice included making a "lean-to" by taking a door off its hinges and placing it against a wall to form a protective shelter, and sending pets outside to test radiation levels.

Mr Eldridge said it was unlikely wearing a gas mask would ensure safety if biological weapons had been released into a particular area.

"By the time the effects of biological weapons are felt, it may be too late," he said.

"If you have no idea when these things have been released, what are you going to do? Go around all day wearing a gas mask? There's nothing people can do at the moment except trust the fact that the authorities would warn them."

The Cabinet Office was recently given responsibility for co-ordinating the response of bodies such as the police and local authorities in the event of a national civil emergency.

"Cross-government contingency plans are in place and regular exercises test responses to a range of different threats, including attacks by chemical or biological weapons," the spokesman said.

He declined to comment further on the nature of any response or the level of threat posed by chemical or biological weapons.