An Iraqi engineer who provided the information that became one of the key planks in the Bush administration's case justifying the invasion of Iraq has been tracked down by undercover reporters to a drab residential block in southern Germany.

Rafid Ahmed Alwan, code-named Curveball (a baseball term for deception), has been in hiding since the invasion five years ago, and lives under an assumed name. He was questioned by German intelligence in the late 1990s when seeking asylum in Germany and told them that he had witnessed a biological weapons programme in Iraq. His "evidence" was made public in a compelling speech to the UN security council by US secretary of state, Colin Powell on 5 February 2003, when he said that Iraq possessed stockpiles of biological weapons that threatened the world and the mobile weapons laboratories to produce them.

Although German intelligence officials had warned the CIA that Curveball's claims were unreliable, and UN inspectors had failed to corroborate them, the Bush administration promoted the existence of such mobile labs for months after the invasion.

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Now Curveball denies having made the claims in the first place. The BBC 2 programme Newsnight broadcast last night secretly filmed footage of the discredited agent who was approached by Der Spiegel magazine in his German hideout where he declined to give a formal interview. His face was blanked out in the footage in which a reporter asked him on his doorstep whether he had ever spoken about Iraq's biological weapons. Curveball replied "No."

Der Spiegel, describing the stocky man with a full shock of black hair and a stubbly beard shown for the first time on television since the war, said it is clear that Curveball "is an impostor, a fabulist." However the magazine also criticised German intelligence for remaining loyal to its source, despite the "serious doubts" over his information.