A former Secret Service agent who guarded the door of the Oval Office during the presidency of Bill Clinton is to publish a book in which he will claim Hillary Clinton "lacks the integrity and temperament" to serve as president.

Gary Byrne described Mrs Clinton as "volcanic, impulsive, and enabled by sycophants".

In extracts from his book, Crisis of Character, he claimed to have "personally observed President Bill Clinton's infidelities".

On one occasion in 1995 he heard Mrs Clinton's "booming voice" from the White House living quarters as the first couple argued, he claimed.

Later, a glass vase was found smashed on the floor and President Clinton had a "put a steak on it" black eye the next day, he wrote.

Mr Byrne said when he asked about the "shiner" he was told by President Clinton's press secretary that it was the result of an allergy to coffee.