The news will come as a blow to US President George Bush and British Prime Minister Tony Blair who based their case for war against Iraq on the "imminent threat" from Saddam Hussein's weapons programmes.
And it was met with anger on Thursday from anti war campaigners who have always doubted the existence of WMD's in Iraq.
"We have not found at this point actual weapons," the expert said after briefing Congress on the work of his team in Iraq since the war that brought down Saddam Hussein.
But he added that, "At this point, we have found substantial evidence of an intent of senior level Iraqi officials, including Saddam, to continue production at some future point in time of weapons of mass destruction."
He said the findings did not mean the United States had concluded there were no weapons.
Kay said experts in the Iraq Survey Group that he heads had found "a large body of continuing activities and equipment that were not declared to the UN inspectors when they returned in November of last year."
This included "substantial equipment and activities in the chemical and biological area, a much more substantial activity in the missile area".
According to Kay, Saddam's regime was carrying out "a very full-scale programme" that would have extended the range of its missiles beyond 1000km, capable of reaching Cairo and Riyadh from Iraqi territory.
Kay estimated it would take between six and nine more months to give a firm indication of the state of the Iraqi weapons programme.
"Believe me, if I wanted to go into business, I would go into the metal detection business in Iraq. I think for 100 years they will be digging up the relics of Saddam's empire that are buried over the country."
He added: "My advice to everyone is still don't be surprised by surprises in Iraq."
Washington-based Nihad Awad, executive director of the Council on American-Islamic Relations (CAIR) said: "The US based its argument for war solely on the WMD issue. This news shows that the evidence was either fabricated or the public was misled.
"The Americans, along with the British, took unilateral action against the will of the international community. These findings expose the neo-Con agenda and the misguided doctrine of preemptive strikes.
"There's no point in dragging this out. The Americans should come clean and admit they've made a mistake. Why are they prolonging this process ... to fabricate evidence?"
Speaking to Aljazeera.net, he added: "Once the final evidence comes out I am sure British and American voters will be watching closely and take note.
"Lots of governments in the world have intent to produce biological and chemical weapons but if we use that as a basis to invade countries then there will never be peace in the world."