Family friend Tara Palmer Tomkinson was treated at a rehab clinic

William's friend Tom Parker Bowles - another godchild and son of Charles's girlfriend Camilla - was exposed as a cocaine user two years ago.

The Prince of Wales was reported to have telephoned the then 24-year-old and given him a severe scolding, calling him a "bloody fool".

He was reportedly banned from seeing William for several months, and ordered to quit drugs if he wanted to remain part of his circle.

His cousin Emma Parker Bowles admitted in 2000 that she had been treated for drink and drug addiction.

She attended the same clinic where Royal family friend and former "It Girl" Tara Palmer-Tomkinson sought refuge in 1999.

'Completely wrecked'

The latter had been battling a problem with "cocaine and champagne", admitting on Frank Skinner's BBC1 TV show in April of that year she was so drunk she did not know who he was.

She was later reported to have been barred from joining the Prince of Wales's summer cruise.

William himself is understood to be against drugs

Now 17-year-old Prince Harry has reportedly smoked cannabis in the grounds of Charles' Highgrove estate in Gloucestershire, and drunk alcohol at a nearby pub.

There have never been any reports of Prince William drinking excessively or taking drugs. He is understood to be against drugs, although there have been reports of a cigarette or two.

Before heading off to university, he joked that he was skipping fresher's week in case he ended up in a gutter "completely wrecked".

Cherry brandy

As for the older generation - Prince Charles, who recently told schoolchildren he had tried smoking but "gave up at the age of 11", was himself once involved in an infamous underage drinking episode.

He was 14 when, during a school sailing trip in Scotland he and four other boys were given permission to dock at the Isle of Lewis for supper and to watch a film.

Charles led the boys to a nearby hotel, where he was surprised to find himself in a public bar. He asked for a drink, and a cherry brandy was the first thing that came into his mind.

The incident attracted international attention. Buckingham Palace staff furiously denied the incident, before admitting it.

Charles has since said the words cherry brandy make him "wince".