Blair followed the US lead by failing to reveal publicly doubts about the quality of intelligence that he had requested to support the case for war, the programme claims.

Sir Richard Dearlove, head of MI6, briefed Blair and a select group of ministers on America’s determination to press ahead with the war nine months before hostilities began.

After attending a briefing in Washington, he told the meeting that war was “inevitable”. Dearlove said “the facts and intelligence” were being “fixed round the policy” by George W Bush’s administration.

The allegations against Blair just weeks before a general election are likely to reopen the feud between the government and the BBC that came to a head over the death of Dr David Kelly, the former weapons inspector. It led to the resignations of Gavyn Davies, its chairman, and Greg Dyke, its director-general.

The documentary — to be shown on BBC1’s Panorama tonight — reveals that Britain and America were anxious to present a united front on Iraq despite a paucity of new data on Saddam Hussein’s weapons of mass destruction (WMD).

It quotes from a leaked memo on the presentation of intelligence sent by Peter Ricketts, political director of the Foreign Office, to Jack Straw, foreign secretary, in March 2002.

The memo says: “There is more work to ensure that the figures are accurate and consistent with the US. But even the best survey of Iraq’s WMD programmes will not show much advance in recent years.”

The programme argues that Blair had signed up to follow Bush’s plans for regime change in Iraq as early as April 2002. It quotes Robin Cook, the former foreign secretary who resigned as leader of the Commons over Iraq, arguing that the threat of WMD was not Blair’s true reason for going to war.

Cook says: “What was propelling the prime minister was a determination that he would be the closest ally to George Bush and they would prove to the United States administration that Britain was their closest ally. His problem is that George Bush’s motivation was regime change. It was not disarmament. Tony Blair knew perfectly well what he was doing.

“His problem was that he could not be honest about that with either the British people or Labour MPs, hence the stress on disarmament.”

The intelligence services had little evidence to show that Iraq was a serious threat. At the meeting with Dearlove in July, Straw was still not entirely convinced. But, the programme claims, Blair had to keep talking up the threat posed by Iraq to justify his policy of supporting Bush. MI6 was then tasked to seek new information from its limited Iraqi network to make the case for war.

The little intelligence that could be gathered was seized upon by Alastair Campbell, Blair’s press secretary, and John Scarlett, the official leading a team drawing up the now notorious intelligence dossier.

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