The "slide to war" with Iraq is the most dangerous crisis since Cuba 40 years ago, veteran Labour MP Tam Dalyell has warned.

Mr Dalyell, father of the House of Commons and leader of the anti-war campaign, told a rally in London's Hyde Park: "We are sleep-walking to disaster."

"The Government's dossier states that Iraq has chemical and biological weapons ready to use," he said.

"We know Iraq was prepared to use them in 1991 if Baghdad was attacked. We can be sure they will use them if cornered."

He claimed that the US was deploying B2 stealth bombers to Diego Garcia. "One of the weapons they can carry is the new B61-11 earth-penetrating nuclear bomb.

"It is designed specifically to destroy underground bunkers, including those holding chemical and biological weapons."

'Nerve gas attack'

Mr Dalyell, MP for Linlithgow, said he asked in the Commons last Tuesday if nuclear weapons were being moved to Diego Garcia but was not given an answer.

"We must demand to know whether the US have moved these weapons to Diego Garcia, a British Indian Ocean territory, and how it would be intended to use them."

Mr Dalyell recalled that the Defence Secretary, Geoff Hoon, had said earlier this year, speaking about Iraq: "They can be absolutely confident that in the right conditions we would be willing to use our nuclear weapons."

But Mr Dalyell asked: "What are the right conditions? Would many hundreds of deaths among British troops in an anthrax or VX nerve gas attack be the occasion for the launch of the Trident missile, the first use of nuclear weapons since Nagasaki?"