Bob Woodward and Carl Bernstein, the reporting team whose Watergate investigation led to the downfall of President Richard Nixon, today broke a 30-year silence to describe the "monumental risk" taken by Deep Throat, the anonymous Whitehouse source whose identity has finally been confirmed.

In an interview with the Washington Post, which carried the original series of articles in the 1970s and of which Mr Bernstein is now assistant managing editor, they said: "We've kept [his] secret because we keep our word."

After years as the best-kept secret in American journalism, Mark Felt, the deputy director of the FBI in the 1970s, finally agreed to being exposed in an article for Vanity Fair.

As politicians and analysts took in the revelation, Mr Bernstein, who has continued to pursue his journalistic career in books and television investigations, said: "Felt's role in all this can be overstated."

"He gave us encouragement," Mr Bernstein writes in The Washington Post today.

"Mark Felt at that time was a dashing grey-haired figure. And he gave Ben comfort," Mr Woodward added, referring to his then executive editor Ben Bradlee who, although aware that the source was a senior FBI official, did not learn Mr Felt's name until after the Post had won the Pulitzer Prize and Richard Nixon had resigned.

Mr Bradlee says in The Washington Post: "The thing that stuns me is that the goddamn secret has lasted this long. What would you think the odds were that this town could keep that secret for this long?"

The Watergate investigation became a book and later a film, All The President's Men, starring Robert Redford and Dustin Hoffman.

Mr Bernstein says he was surprised at the extent to which the legendary shadowy figure of Deep Throat seeped into the public consciousness.

He writes: "When we wrote the book, we didn't think his role would achieve such mythical dimensions. Felt/Deep Throat largely confirmed information we had already gotten from other sources."

Until today only four people have known Deep Throat's identity despite enormous speculation which has generated a journalistic sub-industry in America: Woodward, Bernstein, Bradlee and Deep Throat himself.

Mr Felt, 91, was enjoying his belated moment in the spotlight today. He stood at his front door in Santa Rosa, California, and, bathed in camera flashes, grinned and waved while leaning gingerly on a walking frame.

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