WASHINGTON - Deep Throat's former boss said he was shocked when Mark Felt was revealed as the secret source on the Watergate scandal who helped bring down Richard Nixon's presidency.

Patrick Gray, who was acting FBI director during Watergate, said in an interview with ABC's This Week that Felt had repeatedly told him he was not the source for The Washington Post where reporters Bob Woodward and Carl Bernstein were aggressively breaking stories on Watergate.

"This was a tremendous surprise to me. I could not have been more shocked, and more disappointed in a man whom I had trusted," Gray said of Felt, who was his deputy at the FBI.

Gray said he decided to break three decades of media silence because he was "very ill" and Felt had been revealed as Deep Throat.

Gray said he had not suspected Felt of being Deep Throat.

"He was under suspicion by everyone but his immediate boss, because I was working with the man on a daily basis, and he presented to me a picture of an honourable individual doing his job," Gray said.

The scandal grew after the break-in at Democratic national headquarters at the Watergate office complex in Washington.

But in the end, Gray said that it was White House counsel John Dean talking to the Watergate prosecutors, and not Felt, who brought down Nixon, who in August 1974 became the only US president in history to resign.

Gray said Dean and John Ehrlichman handed him an envelope on June 28, 1972, saying it contained papers removed from Howard Hunt's safe that had nothing to do with Watergate but "must not see the light of day".

He said he did not open the envelope until months later, and found it contained false top secret cables linking the Kennedy administration to the assassination of South Vietnamese President Ngo Dinh Diem in 1963. The second set of papers were apparently false letters that could possibly have been politically damaging to Democratic Senator Edward Kennedy.

Gray said he took the papers to his Connecticut home and burned them.

He also expressed anger at Nixon and said he wished he had never joined that administration. "I made the gravest mistake of my 88 years in making that decision."

Nixon would send him books after Watergate, but Gray said he never responded. "I refused all contact," he said.

"I was so hurt and so angry at this man who had not only junked his own presidency but junked the careers of so many other people, many of whom had to go to jail."