The weapons inspector given the task of finding Saddam Hussein's much-vaunted weapons of mass destruction last night declared that he had found none.

However, David Kay, head of the 1,000 strong CIA-led Iraq Survey Group, said that he had discovered evidence of WMD-related programmes as well as indications that Saddam had remained "firmly committed" to acquiring nuclear weapons.

In his interim report to the US House and Senate Intelligence Committees, Dr Kay said: "We have not yet found stocks of weapons.

"But we are not yet at the point where we can say definitely either that such weapons stocks do not exist or that they existed before the war and our only task is to find where they have gone. We are actively engaged in searching for such weapons based on information being supplied to us by Iraqis."

Foreign Secretary Jack Straw said the report was "further conclusive and incontrovertible evidence" that the Iraqi regime had breached United Nations resolutions.

It showed that the military action taken by the US and the UK was "justified and essential", he said.

But Andrew Murray, chairman of the Stop The War Coalition, said the report confirmed that the reasons given for going to war were false.

"The Prime Minister now owes the nation an apology," he said. "His refusal to do so at this week's Labour party conference and his invitation to President Bush to visit Britain next month will ensure continuing protests from the millions of people who opposed the war."

Dr Kay's Iraq Survey Group (ISG) has been scouring the country for the past three months for the weapons which were the basis for the invasion.

The outcome of their efforts could have a crucial bearing in the political futures of both Prime Minister Tony Blair and - to a lesser extent - President George Bush.

Dr Kay, a former United Nations weapons inspector credited with dismantling Saddam's nuclear programme in the 1990s, stressed that his report was just a "snapshot" of the on-going investigation.

"The report does not represent a final reckoning of Iraq's WMD programmes, nor are we at the point where we are prepared to close the file on any of these programmes," he said.

"While solid progress - I would say even remarkable progress considering the conditions that the ISG has had to work under - has been made in this initial period of operations much remains to be done."

He added: "It is far too early to reach any definitive conclusions and, in some areas, we may never reach that goal."

The ISG's efforts had been hindered by deliberate dispersal and destruction of material on WMD as well as systematic and deliberate looting of sites, he said.

Some WMD scientists had crossed borders and could have taken evidence with them, while the size of any actual WMD material compared to conventional arms made it "difficult to near impossible" to find, he said.

Dr Kay went on to detail the evidence of WMD-related programmes that had been discovered by the ISG.

"We have discovered dozens of WMD-related programme activities and significant amounts of equipment that Iraq concealed from the UN during the inspections that began in late 2002," he said.

These included a clandestine network of laboratories and safehouses that contained equipment suitable for continuing chemical and biological weapons research.

The ISG also found reference to strains of biological organisms concealed in a scientist's home, one of which could be used to produce weapons.