The potentially pivotal role played by George Bush Senior during the Watergate crisis has been disclosed for the first time by his son and fellow former president.

In a new book about his father, published this week, the younger Bush tells how the then-Chairman of the Republican National Committee wrote a private letter to Richard Nixon on August 7 1974 urging him to resign over the scandal.

The president quit the next day and Mr Bush, who went on to become president himself 27 years later, never discussed the letter in public.

The anecdote is told in “41, Portrait of My Father” described by the Drudge Report website, which obtained advance extracts, as a “surprisingly emotional” biography by former President George W Bush about his father and predecessor in the White House.

Called “41” in reference to the senior President Bush’s position as the 41st president of the United States, the book includes the new anecdote about his role in Watergate.

In a chapter called "Diplomacy," the younger President Bush wrote: "The final straw came on August 5, 1974. The Supreme Court had ruled that the White House must turn over all the tapes to Leon Jaworski, the new Watergate special prosecutor and a friend of Dad’s from Houston.

"The tapes revealed that Nixon had spoken to one of his aides about thwarting the FBI’s investigation into the Watergate break-in. That was proof that he knew about the cover-up and that he had lied to the country. The revelation shattered Dad’s trust in Nixon.

"Despite his deep disappointment, my father refused to condemn Nixon publicly.

"While he might have benefited in the short run, Dad saw little point in 'piling on,' as he put it. He voiced his opinion privately in a letter to the President on August 7.

"As far as I know, he is the only party Chairman in American history who has ever written such a letter.

"'I now firmly feel that resignation is best for this country, best for this President,' he wrote. 'I believe this view is held by most Republican leaders across the country.'