It was the Iraqi defector's testimony that led the Bush administration to claim that Saddam had built a fleet of trucks and railway wagons to produce anthrax and other deadly germs.

In his presentation to the UN security council in February last year, the US secretary of state, Colin Powell, explicitly used Curveball's now discredited claims as justification for war. The Iraqis were assembling "mobile production facilities for biological agents", Mr Powell said, adding that his information came from "a solid source".

These "killer caravans" allowed Saddam to produce anthrax "on demand", it was claimed. US officials never had direct access to the defector, and have subsequently claimed that the Germans misled them.

Yesterday, however, German agents told Die Zeit newspaper that they had warned the Bush administration long before last year that there were "problems" with Curveball's account. "We gave a clear credibility assessment. On our side at least, there were no tricks before Colin Powell's presentation," one source told the newspaper.

Officially, Germany's intelligence agency, the BND, has refused to comment.

The revelation is embarrassing for the Bush administration and appears to bolster the contention that it used dubious intelligence in a partisan manner in the critical few weeks before the invasion of Iraq.

It has now emerged that Curveball is the brother of a top aide of Ahmad Chalabi, the pro-western Iraqi former exile with links to the Pentagon.

According to the Los Angeles Times, it was UN inspectors who came up with the idea that Saddam might have developed mobile factories to try to evade weapons inspections. They asked Mr Chalabi, a bitter enemy of Saddam, to find evidence to support the theory.

Recently, American officials have admitted that Curveball's information was false. Meanwhile, David Kay, who resigned as head of the Iraqi survey group in January after a fruitless nine-month search for weapons of mass destruction, said in an interview that Curveball had been "absolutely at the heart of the matter", but had turned out to be an "out and out fabricator".

US and British intelligence officials have acknowledged since the war that much of the information supplied by Mr Chalabi's Iraqi National Congress and other Iraqi groups was wrong. Yesterday, German sources said they were bemused by the idea that they had tricked the US. "We ask ourselves, what are they on about?" one said.

© Guardian Newspapers Limited 2004