WASHINGTON - He was the most famous journalistic source in history and he helped bring down a corrupt US president.

For more than three decades his identity has been one of Washington's most closely guarded secrets.

But now the mystery that has bedevilled amateur sleuths and political pundits for more than 30 years is over. 'Deep Throat' has finally stepped out of the shadows. 

It was confirmed today that Mark Felt, a former senior FBI official, was the source for Washington Post reporters Bob Woodward and Carl Bernstein's Watergate story.

"I'm the guy they used to call Deep Throat," he said in Vanity Fair, and today Woodward and Bernstein confirmed Felt was their source.

Their reporting - much of it based on tips provided by Deep Throat in late night meetings at an underground car-park - revealed President Richard Nixon's involvement in the Watergate affair and other dirty tricks against the Democratic Party.

Nixon was subsequently forced to resign in August 1974.

Ever since the publication of the reporting pair's book All the President's Men, which revealed they had used a secret source but did not identify him, countless others have speculated, investigated and expended endless energy trying to identify the source.

The only other person informed of the identity was the reporters' then editor, Ben Bradlee, and Mr Woodward and Mr Bernstein had always said they would only reveal their source once he was dead.

Mr Felt, now aged 91 and with failing mental health, has long appeared at the top of lists of "possibles". At the time of the break-in to the Watergate headquarters of the Democratic Party he was the deputy associate director of the FBI and would have been kept fully abreast of the investigation.

He also had the motive to turn the spotlight on Nixon's nefarious behaviour. A member of J Edgar Hoover's old guard at the FBI, he had hoped to succeed the old man in 1972.

Instead Nixon appointed a loyalist, Patrick Gray, as acting director.

Mr Felt was also known as one of the few senior FBI officials to return reporters' calls.

Nixon's Chief of Staff, HR Haldeman, told the president that Mr Felt was "most of" the leaks plaguing the White House. But while it has been suggested numerous times that he was Deep Throat, Mr Felt has always denied it.

Mr Felt and his family had cooperated for an article in Vanity Fair which confirms he was the secret source.

It is the first time that a major potential source has claimed to be Deep Throat.