A Lebanese woman who became an agent for the FBI and CIA after faking her US citizenship used her position to gain access to secret files on the terrorist group Hezbollah - the latest security breach to hit America’s top intelligence agencies.

Nada Nadim Prouty, 37, was hired by the FBI and CIA after vetting procedures by America’s most security-sensitive agencies failed to uncover that she had obtained US citizenship illegally through a sham marriage.

Ms Prouty pleaded guilty to naturalisation fraud and unauthorised computer access after it emerged that she accessed restricted FBI and CIA computer records to see if her sister and brother-in-law in Lebanon were linked on US terror lists to Hezbollah.

“It is hard to imaging a greater threat than the situation where a foreign national uses fraud to obtain citizenship and then, based on that fraud, insinuates herself into a sensitive position in the US Government,” said Stephen Murphy, the federal prosecutor who obtained her guilty plea.

The case marks the latest serious security breach to hit the FBI. Last month the Justice Department condemned it for having failed to implement new security measures despite the 2001 arrest of Robert Hanssen, a senior FBI official who spied undetected for Russia for 20 years.

The Justice Department concluded that the FBI was still vulnerable to espionage.

Ms Prouty left Lebanon and arrived in the US in 1989. She entered the sham marriage in 1990 and became a US citizen in 1994.

She passed a lie-detector test before being hired by the FBI – with security clearance – in 1999. She worked in Washington investigating crimes against Americans overseas.

The FBI says that it conducted interviews with her relatives in Beirut as part of the background check, but she was not subject to the more rigourous screening that the FBI said it would adopt after Hanssen’s arrest. The CIA relied on the FBI’s checks when it hired her in 2003.

The charges say that between 2000 and 2003 she scrutinised FBI computers for records on herself, her sister and brother-in-law, on a Detroit FBI investigation into Hezbollah, which is listed by the US as a terrorist organisation. Her relatives were linked to Hezbollah in 2004 after they attended a fundraiser in Lebanon featuring a speech by a senior Hezbollah cleric.

US officials say there is no evidence that she passed secrets on to Hezbollah or worked as a spy on their behalf. She faces six to nine months in jail as part of her plea agreement. Her citizenship has been revoked and she will probably be ordered to leave the US after her prison term.

The FBI computer network she accessed is ageing and has come under growing criticism. The agency has failed to replace the system despite several attempts, and concedes that it is unlikely to get a modern one for several years.

Court papers say that Ms Prouty first came under suspicion in 2005 and was under investigation for two years. She came under scrutiny because of an investigation into her brother-in-law, Talal Khalil Chahine, a former Detroit restaurant owner, who is wanted in the US on tax evasion, bribery and extortion charges. He is now a fugitive. Part of the investigation into him centred on a scheme to channel millions of dollars from his restaurant to people in Lebanon.

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