Saddam Hussein allowed the world to believe that he had weapons of mass destruction because he feared the threat of a nuclear-armed Iran more than an American strike, according to declassified documents records of his interrogation.

Saddam said that the dramatic increase in Iran’s weapons capabilities had left him feeling so vulnerable to the “fanatic” mullahs next door that he would have been prepared to forge a security agreement with the United States to protect Iraq from regional threats. He warned that the effects of Iran’s unchecked rise “will be seen and felt in the future”.

The revelation, contained in accounts by George Piro, Saddam’s Arabic-speaking FBI interviewer, declassified this week after a freedom of information request, supports the view of many regional experts that Saddam’s weapons bluff was aimed not at the West but at his old enemy Iran.

Many analysts believe that the removal of Iraq as a bulwark against Iran has merely fuelled Tehran’s rise to its current hegemony in the Middle East. Iran and Iraq fought a devastating eight-year war from 1980 to 1988, in which Saddam was supported and armed by the United States.

“The threat from Iran was the major factor as to why Hussein did not allow the return of the UN inspectors,” Mr Piro wrote. “Hussein stated he was more concerned about Iran discovering Iraq’s weaknesses and vulnerabilities than the repercussions of the United States for his refusal to allow UN inspectors back into Iraq.”

He acknowledged that, in retrospect, it was an error not to allow weapons inspectors to witness the destruction of Iraq’s weapons stockpile after the Gulf War in 1991. This lack of verification was a key factor in the American and British insistence that dangerous weapons caches remained.

“Hussein further stated that Iran’s weapons capabilities have increased dramatically, while Iraq’s have been eliminated by the UN sanctions,” Mr Piro wrote after an informal conversation in Saddam’s cell at Baghdad airport in June 2004.

Saddam said that, until four months before the American invasion, he believed that any reprisals would take the form of limited military strikes such as the four-day bombardment during Operation Desert Fox in 1998, which he was confident that Iraq could absorb.

The interviews took place between January and June 2004, with Saddam in turns boastful, defiant, regretful and resigned. One allegation appeared to aggrieve him particularly: the Bush Administration’s insistence that he was in league with al-Qaeda. Saddam scoffed at this, dismissing Osama bin Laden as a “zealot”. Saddam said that he was “a believer in God but religion and government should not mix”.

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