The heads of MI6, MI5 and GCHQ will give evidence to the Commons Intelligence and Security Committee, which is investigating the issue.

Tony Blair, who has called for patience in the search for WMD, has been told that rocket motors for long-range missiles have been discovered in Iraq, The Times understands. Senior Whitehall sources said that in the past few weeks US and British weapons experts discovered that the Abu Ghraib military base was developing a weapon with a range of about 960km (600 miles), capable of reaching Israel and other parts of the Middle East. Under limits set by the UN Security Council, Iraq was allowed to missiles with a range of up to 150km only.

The heads of the intelligence services sit on the Joint Intelligence Committee and all “signed off” the dossier published last September. They are said to remain “quietly confident” that their assessment of Saddam’s weapons programme will be vindicated.

Officials within the intelligence community said that the chiefs would have no problem about giving evidence to an inquiry. John Scarlett, the former MI6 station chief in Moscow who chairs the Joint Intelligence Committee, has also indicated his support and dismissed suggestions of hype.

One senior Whitehall official acknowledged yesterday that ministers had seized on certain aspects of the dossier to emphasise the perceived Iraqi threat. “That is what politicians do, but it does not undermine either the facts or the judgments that appeared in the dossier,” the official said.

The Prime Minister has hinted that further evidence of Saddam’s secret programme has begun to emerge but has yet to be made public. However, Whitehall sources said: “What we need to find is firm evidence of a chemical weapons programme, because much emphasis was placed on the chemical threat. We need to discover why Saddam did not use chemical weapons against coalition forces.”

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