Former Chief of Defence Intelligence Sir John Walker said information produced by the agencies normally guided government policy. But in the months leading up to the war, that principle had been reversed, he said.
"It seems to me that policy was driving intelligence and that is an extremely dangerous thing to do as a nation-state," Walker told BBC radio.
Walker left the Ministry of Defence in 1995, two years before Blair was elected.
Blair's government is braced for a potentially damaging report on the quality of British intelligence on Iraq, due to be published by retired civil service chief Lord Butler on Wednesday.
In the run up to the war, Blair was adamant that Iraq possessed stockpiles of chemical and biological weapons and was developing a nuclear weapon.
In a dossier published by the government in September 2002 , Blair wrote that intelligence had established "beyond doubt" that Saddam continued to produce chemical and biological weapons.
The dossier was central to the British government's case for invading and occupying Iraq.
Lack of evidence
However, the agency charged with unearthing Iraq's alleged illicit weapons, the Iraq Survey Group, has been unable to substantiate any of the pre-war claims made by both Washington and London.
"I have never in my experience come across the JIC being used in this way before. It was clearly way outside the normal way in which the JIC operates"
Sir John Walker,
Former chief of UK Defense Intelligence
Butler's inquiry aims to establish why there is such a gap between intelligence "gathered, evaluated and used by the government" and the lack of evidence on the ground in Iraq.
The September dossier was drawn up by Britain's Joint Intelligence Committee (JIC) which assesses raw intelligence.
Committee chairman John Scarlett signed off on the document before it was published by Blair's government.
Walker, a former deputy chairman of JIC, said "I have never in my experience come across the JIC being used in this way before. It was clearly way outside the normal way in which the JIC operates".
"The normal run of the system should be that intelligence produces the information that it can from the information that it has and produces that information to government and policy-makers. It is intelligence into policy," he told the BBC.
"The thing that does worry me about this, because of the dire results of it - let's face it, the nation went to war - is that [the system] was reversed," he added.
Three previous inquiries have cleared the government of acting dishonestly or misusing the intelligence made available to it to bolster the case for toppling Iraqi President Saddam Hussein.
But a parliamentary inquiry last year said intelligence assessments failed to reflect "the uncertainties and gaps in the UK's knowledge about the Iraqi biological and chemical weapons."
In recent months, Blair has retreated from his confident assertions that Iraq had stockpiles of illicit weapons to the extent that he said weapons of mass destruction might never be found.
Robin Cook says he told Blair that
Saddam had no WMD's
"I have to accept that we have not found them and that we may not find them," he told a House of Commons committee last week.
"We do not know what has happened to them, they could have been removed, they could have been hidden, they could have been destroyed."
Former foreign secretary Robin Cook said on Monday that Britain's top intelligence official and Blair had told him before the war that Iraq did not have any usable weapons of mass destruction.
The then-head of the Joint Intelligence Committee (JIC) John Scarlett informed Cook of Iraq's weapons capabilities in February 2003, a month before the start of the war, the former foreign minister said in an extract of his memoires published in Britain's Guardian newspaper on Monday.
Cook, who was leader of the House of Commons, talked with Scarlett for nearly an hour about the information possessed by the intelligence services at the time.
"When I put to him my conclusion that [Iraq] had no long-range weapons of mass destruction but may have battlefield chemical weapons, he readily agreed," Cook said.
He continued: "When I asked him why we believed Saddam would not use these weapons against our troops on the battlefields, he surprised me by claiming that, in order to evade detection by the UN inspectors, Saddam had taken apart the shells and dispersed them, with the result that it would be difficult to deploy them under attack.
"Not only did Saddam have no weapons of mass destruction in the real meaning of that phrase, neither did he have usable battlefield weapons," wrote Cook, foreign secretary from 1997-2001.
"When I put to him my conclusion that Saddam had no long-range weapons of mass destruction but may have battlefield chemical weapons, he readily agreed"
Former UK foreign secretary
He said he had put these points to the prime minister two weeks later on 5 March 2003.
"Tony Blair gave me the same reply as John Scarlett, that the battlefield weapons had been disassembled and stored separately," he said.
Cook said he was therefore "mystified" to hear him say a year later that he had never understood that the intelligence services did not believe Saddam had long range weapons of mass destruction.
Cook resigned as House of Commons leader on 17 March 2003 to show his opposition to the war in Iraq. Scarlett for his part was promoted in May to the head of the foreign intelligence service MI6.
Blair took Britain, unauthorised by the UN, into the US-led war to invade Iraq almost exclusively on the assertion that Iraq's alleged stocks of chemical and biological weapons posed a "severe threat" to the world.