Tony Blair last night urged Saddam Hussein to take the opportunity to give up his weapons of mass destruction as America described the discovery in Iraq of chemical warheads as "troubling and serious".

The Prime Minister was briefed for an hour yesterday at his official country residence, Chequers, by the United Nations chief weapons inspector Hans Blix on the progress his team was making.

Afterwards, Downing Street made no reference to the discovery of the 12 empty warheads, but expressed Mr Blair's "strong support" for Dr Blix and the inspection process.

"He welcomed the continuing build-up in the intensity of the inspection programme as more inspectors arrive in Iraq and as their resources improve," said Mr Blair's official spokesman.

His comments will be seen as a signal that Britain would support a request from Dr Blix for more time to carry out inspections when he reports to the UN Security Council on January 27.

Mr Blair's spokesman again played down the January 27 meeting, simply describing it as an "update".

In Washington, White House spokesman Ari Fleischer voiced concern at the discovery of the warheads, which he said that the Iraqis had failed to disclose in their 12,000 page declaration to the UN last month.

However, he stopped short of describing the cache as a "material breach" of UN Security Council resolution 1441 setting out the conditions for Iraq to give up its remaining chemical, nuclear and biological programmes.

Dr Blix, who earlier held talks with French President Jacques Chirac in Paris, said that he was "not so worried" that the Americans would use the find as an excuse to launch immediate military action.

However, he also issued a sharp warning to Baghdad that it must do more to co-operate with the inspectors if it wanted to avoid conflict.

The message was echoed in Downing Street, where Mr Blair's spokesman said: "The Prime Minister stressed how important it was for Saddam to take full advantage of this opportunity to disarm".

In recent days, Britain and the US have been stressing it was not enough for the Iraqis not to obstruct the work of the inspectors and that they must make a full declaration of their remaining weapons programmes.

In Washington, there has been growing impatience with the inspection process among the more hawkish elements of the US administration.

The hawks want to get military action under way before the fearsome Iraqi summer begins in late April.

Dr Blix is expected make clear to the Iraqis that time was running out for them to start co-operating fully with the inspectors when he holds talks with senior officials in Baghdad on Sunday.