Scotland Yard declined to comment on reports in a Sunday newspaper that the Tube network had been the target.

The men, who are of North African origin, were charged under Section 57 of the Terrorism Act 2000 for the possession of articles for the preparation, instigation and commission of terrorism acts.

The trio are Rabah Chekat-Bais, 21, Rabah Kadris, in his mid-30s, and Karim Kadouri, 33, all of no fixed abode.

We get an awful lot of intelligence which we have to make judgements about

John Prescott

The Sunday Times speculated they planned to release cyanide gas on the London Underground.

But police said no cyanide or noxious gas had been found on them when they were arrested on 9 November.

Deputy Prime Minister John Prescott played down the significance of the arrests, saying they had "excited the press".

He told the BBC: "As the prime minister reminds us we get an awful lot of intelligence which we have to make judgements about.

"In this case it does not appear there is any evidence whatsoever there was going to be a gas attack or use of bombs regarding the three people who have been arrested."

'Vigilant'

But he added the general terrorist threat remains high.

"If it is a direct threat that we know and can prove then the public will know about it," he said.

"But at the same time we keep up a very strong alert. Our forces, police, intelligence services are working flat out to make sure we have the highest possible safety for our citizens."

Unemployed Chekat-Bais appeared before Bow Street Magistrates' Court, in central London, last Monday, 11 November, and Kadris and Kadouri, also both unemployed, appeared in court on Tuesday, 12 November.

All three men were remanded in custody to appear before magistrates again on Monday.

Warning

A spokeswoman from London Underground would not comment on the report but said "all our staff are well trained to look out for the unusual".

Prime Minister Tony Blair, who this week warned the public to be vigilant for any terrorist attacks, has been kept informed by police.

A Home Office spokesman said: "The prime minister has made it clear that there are threat reports every day and these are assessed.

"If the government or police thought it was necessary to give the public a specific warning about any venue, including the Underground, it would do it without hesitation."

'Dirty bomb'

Last week, there was confusion over the extent of Home Office fears over a chemical attack on the UK.

A warning of a possible chemical or nuclear terrorist attack on the UK using a "dirty bomb" or poison gas, was quickly withdrawn and replaced with more general advice.

Former intelligence officer Charles Shoebridge told BBC News 24: "The intelligence services are getting good and useful information and they are able to work on it in a competent and forthright fashion."

Shadow Home Secretary Oliver Letwin said it was strange that it had taken so long for news of the arrests to become public.

"I shall be asking for a full statement from ministers in Parliament," he added.