The foreign secretary has moved to downplay expectations about finding evidence of Iraq's weapons of mass destruction.

Jack Straw said on Sunday he was not sure what would be found by the Iraq Survey Group, the international body charged with uncovering Saddam Hussein's chemical and biological warfare programmes.

Speaking on the BBC's Breakfast with Frost programme, Straw warned he would be wrong to make exact predictions.

"I can't say precisely what will be discovered. No one can say that," he said.

The government has repeatedly asked for patience while the survey group carries out its work.

But until now ministers have firmly expressed their confidence that at least evidence of a weapons programme would be found.

However Straw maintained that he still believes military action to overthrow Saddam had been necessary because of Iraq's past record of defiance of the international community.

"What I can say with absolute certainty is that the decision to go to war, which the House of Commons made by a large majority, was justified," the foreign secretary said.

And he confirmed he had been working with his counterpart in the United States to extend the role of the United Nations in Iraq in the wake of attacks on coalition forces.

"It [the US-UK coalition] is not losing its grip. The truth is that August has been a very bad month for the security situation across Iraq. It is very frustrating for everybody involved," he said.

"What the terrorists are trying to do is to destroy that and that requires both a security and a political response by the US-UK and by the international community.

"It looks as though - although we haven't got really definitive information about this - that the terrorist activity comes from both the extremists associated with Saddam and external infiltrators associated with extremist terrorist organisations working alongside simply criminal gangs."

As Britain takes over the rotating chairmanship of the UN Security Council on Monday, Straw expressed confidence that agreement could be reached on a new resolution to "deepen and strengthen" the UN's mandate in Iraq.

"What I think is that there is a mood in the international community by which we will be able to achieve a consensus within the United Nations," he said.

The Liberal Democrat foreign affairs spokesman Menzies Campbell said that the latest disclosures further undermined the Government's case for war.

"The case for the 45 minute claim is simply withering on the vine," he said.

"We now know that the intelligence was based on hearsay and that any weapons could have only been used on the battlefield. This hardly amounted to an imminent threat."