In an emergency statement to the House of Commons, Mr Cook revealed that French and Swiss laboratories had backed up claims by the United States that the chemical weapons were being actively developed by Iraq.
President Saddam Hussein has repeatedly denied that his military has achieved a deliverable VX missile, but the new evidence shows that his officials had even tried to decontaminate the warheads.
Mr Cook said Britain had to make clear to the Iraqi dictator that it was prepared to use force to ensure compliance with United Nations resolutions on weapons inspections.
He added that Iraq's refusal last week to allow Unscom inspectors to monitor weapons sites was a clear breach of the deal reached in February with Kofi Annan, the UN Secretary-General.
"Saddam Hussein appears to be gambling that the world will grow weary of his constant evasion and his repeated confrontation," Mr Cook said.
"His calculation is that we will eventually give up and abandon the sanctions regime without requiring him to abandon his ambitions for regional supremacy through weapons of terror. We must remain ready and resolute to prove him wrong."
Mr Cook confirmed that Britain still has a 12-strong force of Tornado aircraft on standby in the region, alongside other Allied weaponry, and he was confident that it had a UN mandate to strike. "It would be too dangerous for Iraq's neighbours in the region to leave Saddam Hussein with the capacity to produce weapons that could wipe out whole cities," he said.
"And it would be too damaging to the authority of the UN if Saddam was allowed to break the agreement he entered into with the Secretary-General."
Mr Cook rejected Iraqi claims yesterday that four of the UN inspectors were spies for Mossad, the Israeli intelligence service.
Earlier, the Secretary of State for Defence, George Robertson, met his US counterpart, William Cohen, in London to discuss possible military options.
Mr Robertson said after the meeting that the two nations agreed the threat of force was needed in response to the Iraqi leader's "open defiance" of UN resolutions. "Saddam has got to get a very clear and unanimous message from the international community that this is not going to be tolerated," he said.
The shadow foreign secretary, Michael Howard, gave his support for a strong stand against Iraq, but said he hoped that Mr Cook's warnings were more effective than his "final warnings" to the Serbian President Slobodan Milosevic during the Kosovo crisis in the summer.Reuse content