As the hunt continues for Iraq’s alleged stockpiles of chemical and biological weapons — the main justification cited by President Bush for the war — carefully placed leaks revealed deep misgivings inside the CIA over intelligence used by the White House to make its case against Saddam.

Present and former CIA officials, quoted in The New York Times and The New Yorker magazine, claimed that a small number of powerful neo-conservative ideologues in the Pentagon were so determined to prove the existence of a banned weapons programme and links to al-Qaeda that they manipulated intelligence.

According to a report written by Seymour Hersh, the veteran New Yorker investigative reporter, the Pentagon’s Office of Special Plans (OSP) relied too heavily on suspect intelligence provided by Iraqi defectors with links to the Iraqi National Congress, an opposition group headed by Ahmad Chalabi, an Iraqi exile.

Mr Hersh reported that intelligence gathered by the OSP drove the war agenda, often in the face of evidence that it was either unreliable or false. The OSP reported to Paul Wolfowitz, the Deputy Defence Secretary and a leading proponent of the war.

One former CIA official told Mr Hersh: “One of the reasons I left was my sense that they (OSP) were using the intelligence from the CIA and other agencies only when it fits their agenda. They were so crazed and so far out and so difficult to reason with . . . as if they were on a mission from God. If it doesn’t fit their theory, they don’t want to accept it.”

Mr Hersh maintained that key intelligence provided by Iraqi defectors with links to the Iraqi National Congress was disputed by the CIA.

He cited as one example the evidence of Adnan Ihsan Said al-Haideri, a civil engineer who fled Iraq in 2001 with the National Congress’s help. Mr al-Haideri, Mr Hersh said, was apparently the source for the assertion by Colin Powell, the Secretary of State, in his speech to the UN Security Council that the US had “first-hand descriptions” of hidden mobile chemical and biological factories. The 20 claimed sites have been examined by UN inspectors and US forces and no trace of banned weapons facilities was found.

Patrick Lang, a former head of Middle Eastern affairs in the Pentagon’s Defence Intelligence agency, told Nicholas Kristof, of The New York Times, that when experts wrote reports sceptical about the existence of weapons of mass destruction “they were encouraged to think it over again”.

Saddam Hussein is alleged to call on Iraqis to rise up against US-led forces in Iraq in a cassette said to have been recorded this week, an Australian newspaper reported.

In the recording, delivered to Sydney Morning Herald journalists in Baghdad, a “tired-sounding” voice said: “We have to go back to the secret style of struggle that we began our life with . . . your main task is to kick the enemy out from our country.” The newspaper played the tape to 13 Iraqis, including a former acquaintance of Saddam, and “the overwhelming opinion was that the voice and rhetoric were very similar, or identical, to those of Saddam”.