He delivered his grim warning as he mounted his strongest defence so far of preparations for war, underlining the link between terrorism and weapons of mass destruction. Mr Blair was asked about the threat from al-Qaeda when he appeared before the Commons Liaison Committee, which is made up of select committee chairmen.

He said: “I think it’s inevitable that they will try in some form or other and I think we can see evidence from the recent arrests that the terrorist network is here, as it is around the rest of the world.”

It was the first time that Mr Blair had suggested that an attack was unavoidable. Recent attacks in Bali and Kenya and arrests of activists in several European countries showed that al-Qaeda was ready to commit outrages anywhere it thought it could, he said.

“If we end up in a situation where there is a potential nuclear conflict, every country in the world is going to be drawn into that in some way,” Mr Blair said. “That is why I think there is no point in us thinking — and I don’t think it is actually particularly in the British character to think — well, let’s go to the back of the queue and hide away.”

He suggested that there were only loose links between al-Qaeda and President Saddam Hussein’s regime in Iraq. “I think it’s important we do everything we can to show people the link between weapons of mass destruction and these terrorist groups,” he said.

Mr Blair said that the security and intelligence services were the first line of defence against the terrorist threat. The Government was spending hundreds of millions of pounds on vaccines, protective clothing and new procedures in preparation for an attack.

“We could spend billions of pounds doing it, we could spend tens of billions of pounds doing it, and we could still not identify where the attack was going to come from,” he said. “There are no limits to the potential threats you can imagine. That is why I think the overall part of this is to make every aspect of our security and intelligence information services work as effectively as possible.

“There is a limit to what you can do to prepare yourselves, but we have to do everything we possibly can, and will do. We should make whatever investment is necessary on the basis of the advice we are given. It’s very, very difficult.”

Mr Blair told the committee that he believed that Saddam retained a chemical and biological weapons capability and was trying to rebuild his nuclear weapons programme. The Iraqi leader was hiding those weapons and the missiles that could deliver them.

Mr Blair said that his role over recent months had been to construct the broadest possible international consensus over Iraq and he admitted that it would be “tough” to hold it together over the coming months. He said that it would be “highly desirable” to have a second resolution on Iraq. He added, however: “Of course it’s better if we can go down the UN route, but we must not give a signal to Saddam that there is a way out of this.”

He did not believe that Russia, China or France would use their veto if action was necessary. It was unthinkable, he said, because the spirit of the first resolution was that, if there were findings by the inspectors that amounted to a breach, action would be authorised. “Otherwise the will of the United Nations is set at nought,” he said.

Asked about suggestions that Saddam should be allowed to go into exile, Mr Blair replied that it would be a “great thing” if he were to leave Iraq. “The world, the region, Iraq would be a better place without Saddam and there is no doubt at all that it is Saddam and his immediate entourage who are insistent on keeping the weapons of mass destruction, because they believe that is one of the ways in which they can repress their local population and retain power in the region.

“Of course you would not instinctively oppose it. But, having said that, I don’t know that that is going to happen.”

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