He also had an enthusiasm for service that was becoming rare.

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Late one night, as she crept along the historic corridors of the Queen Mother's official residence, she was horrified to bump into a well-dressed young man casually inspecting one of the paintings.'I'd never seen him before,' she recalled.

'For there to be a stranger alone in the house at night was a serious security breach.

Relying entirely on force of character, he was like a private in the army who becomes a field marshal: the last person one would have expected to reach such a position of influence in a world where education and social class meant everything.

Until her death in 2002, there was only one figure who rivalled the Queen for the world's interest and the nation's affection, and that was her mother, Elizabeth.

But then he always would simply float above any difficulty or embarrassment.'In truth, such a flagrant compromise of security was far from an exception. But the extent to which he used Clarence House for sexual assignations with strangers – and used it as a lure for prospective partners in the gay bars of Soho – has never fully been grasped.

Only now, after countless interviews with former staff and friends, can his extraordinarily risky indiscretions be revealed.

Billy's reputation for loyalty was partly based on his personality, but also because he was gay.

The assumption was that he would have none of the distractions of someone heterosexual, who might eventually want his own family.

Now, for the first time, a new biography tells the remarkable story of a ruthless predator whose outrageous lifestyle included gay orgies at Clarence House, rampant affairs below stairs, sex on the Queen Mother's favourite sofa – and young men wandering the corridors at night...