David Murfee Faulk said he saw a file on Mr Blair in 2006 which contained information of a "personal nature" gleaned from eavesdropping on his private telephone conversations.

The accusation that the Bush regime was spying on its closest ally in the war on terror is particularly embarrassing as there is understood to have been an understanding between the two countries that such espionage would not be carried out.

Mr Faulk said he had top secret clearance as a US Army Arab linguist at a covert giant listening post run by the National Security Agency.

He was able to gain access to an intelligence data base of phone call recordings and transcripts, code-named Anchory, where he found a file on Mr Blair.

He made his disclosures to an investigation by ABC News a month after his claims that the NSA spy on American citizens prompted a Senate investigation.

While it is often claimed spying on allies is routine in international espionage, the closeness of the Bush-Blair alliance meant it was supposed to have been an exception.

According to former US intelligence officials, there has been a long-standing agreement with Britain not to collect such information on each other's leaders.

"If it is true that we maintained a file on Blair, it would represent a huge breach of the agreement we have with the Brits," a former CIA official told ABC News.

A spokesman for Mr Blair refused to comment on the claims.

Mr Faulk declined to say exactly what he saw in Mr Blair's file but said it was information of a "personal nature".

He also revealed that the US was spying on Ghazi al-Yawer, Iraq's first interim president after the US invasion, and another key ally.

He said he heard "pillow talk" phone calls of Mr Yawer in which he chatted with his fiancé, whom he later married.

Mr Faulk, 39, broke his silence about his work at the NSA base at Fort Gordon, Georgia, last month when he claimed that it intercepted the private phone calls of American journalists, aid workers and soldiers stationed in Iraq.

His claims prompted calls by some US congressmen for an investigation into what they described as "extremely disturbing" allegations.

President Bush has previously denied that the NSA has eavesdropped on American citizens and Mr Faulk's insistence to the contrary is to be investigated by the Senate Intelligence and Judiciary Committees.

A second whistle-blower, Adrienne Kinne, has supported his claims about US journalists, saying their conversations were intercepted when they used satellite phones.

Fort Gordon is the world's biggest listening post. Nicknamed Black Hall, it is home of the US Army's Signals Centre and employs some 30,000 intelligence gatherers.

While running the NSA, Gen Michael Hayden, now the CIA director, told Congress that the organisation's eavesdropping targets were carefully chosen.

"It's not for the heck of it. We are narrowly focused and drilled on protecting the nation against al Qaeda and those organisations who are affiliated with it," he said.