According to Nato sources, the investigation has evidence that Niger’s consul and its ambassador’s personal assistant faked a contract to show Saddam Hussein had bought uranium ore from the impoverished west African country.

The documents, which emerged in 2002, were used in a US State Department fact sheet on Iraq’s weapons programme to build the case for war. They were denounced as forgeries by the International Atomic Energy Agency (IAEA) shortly before the 2003 invasion.

The revelation spawned a series of conspiracy theories, most alleging that the British, Italians, or even Dick Cheney, the American vice-president, had had a hand in forging them to back the case for war.

The story was still reverberating around Washington last week with claims that President George W Bush had authorised the leaking of the identity of a CIA agent whose husband cast doubt on the Niger link.

According to the sources, an official investigation believes Adam Maiga Zakariaou, the consul, and Laura Montini, the ambassador’s assistant, known as La Signora, forged the papers for money.

They allegedly concocted their scheme as reports reached western intelligence agencies, including MI6, that Saddam Hussein had been trying to buy uranium ore, known as yellowcake, from Niger. The agencies had no evidence he had succeeded. The pair are alleged to have copied a real contract to look like an agreement with Iraq under which Niger would supply Saddam with 500 tons of yellowcake.

The story of the fake deal had begun with a meeting in a Rome bar in February 2000 set up by Antonio Nucera, an officer in the Sismi, the Italian intelligence agency, between two of his former agents, Rocco Martino and Montini.

However, unknown to the Sismi, Martino, a former policeman turned spy, had been working for the French intelligence service, the DGSE, since 1996. He was controlled by the DGSE head of station in Brussels, who paid him a retainer of between £1,050 and £1,400 a month.

“Nucera asked if I was interested in meeting a person who worked in an African embassy and who had been able to supply [Nucera with] documents and information, including the embassy’s cipher,” Martino told an investigating magistrate during an Italian inquiry.

Montini is understood to have agreed to work for Martino, who paid her £350 a month as a “sub-agent”.

In the spring of 2000, she handed him a document relating to a visit to Niger by Wissam al-Zahawie, the Iraqi ambassador to the Vatican. Martino passed it to his French handler.

The French, who were watching for an attempt by Saddam to obtain uranium from Niger, showed great interest and told Martino they wanted more information. Martino asked Montini if she could get a copy of a contract for Niger to supply Iraq with uranium.

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