Man codenamed Curveball 'invented' tales of bioweaponsIraqi told lies to try to bring down Saddam Hussein regimeFabrications used by US as justification for invasion

The defector who convinced the White House that Iraq had a secret biological weapons programme has admitted for the first time that he lied about his story, then watched in shock as it was used to justify the war.

Rafid Ahmed Alwan al-Janabi, codenamed Curveball by German and American intelligence officials who dealt with his claims, has told the Guardian that he fabricated tales of mobile bioweapons trucks and clandestine factories in an attempt to bring down the Saddam Hussein regime, from which he had fled in 1995.

"Maybe I was right, maybe I was not right," he said. "They gave me this chance. I had the chance to fabricate something to topple the regime. I and my sons are proud of that and we are proud that we were the reason to give Iraq the margin of democracy."

The admission comes just after the eighth anniversary of Colin Powell's speech to the United Nations in which the then-US secretary of state relied heavily on lies that Janabi had told the German secret service, the BND. It also follows the release of former defence secretary Donald Rumsfeld's memoirs, in which he admitted Iraq had no weapons of mass destruction programme.

The careers of both men were seriously damaged by their use of Janabi's claims, which he now says could have been – and were – discredited well before Powell's landmark speech to the UN on 5 February 2003.

The former CIA chief in Europe Tyler Drumheller describes Janabi's admission as "fascinating", and said the emergence of the truth "makes me feel better". "I think there are still a number of people who still thought there was something in that. Even now," said Drumheller.

In the only other at length interview Janabi has given he denied all knowledge of his supposed role in helping the US build a case for invading Saddam's Iraq.

In a series of meetings with the Guardian in Germany where he has been granted asylum, he said he had told a German official, who he identified as Dr Paul, about mobile bioweapons trucks throughout 2000. He said the BND had identified him as a Baghdad-trained chemical engineer and approached him shortly after 13 March of that year, looking for inside information about Saddam's Iraq.

"I had a problem with the Saddam regime," he said. "I wanted to get rid of him and now I had this chance."

He portrays the BND as gullible and so eager to tease details from him that they gave him a Perry's Chemical Engineering Handbook to help communicate. He still has the book in his small, rented flat in Karlsruhe, south-west Germany.

"They were asking me about pumps for filtration, how to make detergent after the reaction," he said. "Any engineer who studied in this field can explain or answer any question they asked."

Janabi claimed he was first exposed as a liar as early as mid-2000, when the BND travelled to a Gulf city, believed to be Dubai, to speak with his former boss at the Military Industries Commission in Iraq, Dr Bassil Latif.

The Guardian has learned separately that British intelligence officials were at that meeting, investigating a claim made by Janabi that Latif's son, who was studying in Britain, was procuring weapons for Saddam.

That claim was proven false, and Latif strongly denied Janabi's claim of mobile bioweapons trucks and another allegation that 12 people had died during an accident at a secret bioweapons facility in south-east Baghdad.

The German officials returned to confront him with Latif's version. "He says, 'There are no trucks,' and I say, 'OK, when [Latif says] there no trucks then [there are none],'" Janabi recalled.

He said the BND did not contact him again until the end of May 2002. But he said it soon became clear that he was still being taken seriously.

He claimed the officials gave him an incentive to speak by implying that his then pregnant Moroccan-born wife may not be able to travel from Spain to join him in Germany if he did not co-operate with them. "He says, you work with us or your wife and child go to Morocco."

The meetings continued throughout 2002 and it became apparent to Janabi that a case for war was being constructed. He said he was not asked again about the bioweapons trucks until a month before Powell's speech.

After the speech, Janabi said he called his handler at the BND and accused the secret service of breaking an agreement that they would not share anything he had told them with another country. He said he was told not to speak and placed in confinement for around 90 days.

With the US now leaving Iraq, Janabi said he was comfortable with what he did, despite the chaos of the past eight years and the civilian death toll in Iraq, which stands at more than 100,000.

"I tell you something when I hear anybody – not just in Iraq but in any war – [is] killed, I am very sad. But give me another solution. Can you give me another solution?

"Believe me, there was no other way to bring about freedom to Iraq. There were no other possibilities."