The US and UK are accused of relying on questionable information that suggested Saddam Hussein was manufacturing weapons of mass destruction (WMD), despite warnings over its authenticity.

At the same time, other foreign intelligence that suggested no such programme existed was dismissed, according to a BBC Panorama investigation. One Iraqi spy - codenamed "Curveball" - whose claims to have witnessed the manufacture of WMD were seized upon by the Americans told the programme the invasion had been based on his "lie".

Lord Butler, who, a year after the invasion, carried out a British review of the intelligence used, admitted that he was unaware that two senior members of Saddam's regime had secretly told the CIA and MI6 that WMD did not exist. The documentary reignites questions over the legality and justification of the Iraq war. One source told the programme the conflict was borne out of "choice" rather than "necessity". The war, which started on March 20, 2003, lasted more than six years, claimed the lives of 179 UK personnel, more than 100,000 Iraqis, and cost more than £9 billion.

Britain ended combat operations there in 2009, but the war remains unfinished business, with questions remaining about the legality of the invasion and the conduct of British troops. Tonight's Panorama: The Spies Who Fooled The World, reports how claims from a few sources that Iraq was manufacturing WMD spiralled into apparently sound intelligence that was used to justify the war. In his first TV interview on the subject, August Hanning, the former head of German intelligence, said that Curveball, whose real name is Rafed Al Janabi, told the German secret services that he had witnessed the manufacture of chemical and biological weapons.

The information was passed by the Germans to American and British intelligence, along with concerns about its reliability, he said. Tyler Drumheller, the former head of the CIA in Europe, also claimed that he passed warnings about Curveball's claims up the chain of command, while Mr Hanning said he sent a cable to George Tenet, who was then director of the CIA. Mr Tenet denies receiving the warnings, the programme said.

Joschka Fischer, the former German foreign minister, said US colleagues were warned about the information. However, he told Panorama: "They were not in a state, or a mindset to be warned." Peter Taylor, a Panorama reporter, asked Curveball: "The fact is we went to war in Iraq on a lie. And that lie was your lie." "Yes," he replied, smiling.

A claim from another source given to French security services, which suggested that Iraq was buying large amounts of uranium ore, is said to have spread like "contagion" through the intelligence community. Pierre Brochand, the former head of France's foreign intelligence agency, told the programme: "We felt that intelligence was used to justify a war, which was a war of choice, of pure choice.

But the intelligence was used to disguise that as a war of necessity." Panorama heard how Iraq's foreign minister, Naji Sabri, and head of intelligence, Tahir Habbush al-Tikriti, separately revealed to the CIA and MI6 that there was no active WMD programme, but that information was not given the same credence. Lord Butler said he did not know about Habbush's evidence until after the inquiry and never knew about Sabri's comments.