BBC Royal Correspondent Jennie Bond: "Tom Parker Bowles is keeping a low profile"

The 24-year-old film publicist and son of Camilla Parker Bowles received the call while on a business trip to Cannes in France.

Prince Charles, whose sons William and Harry are good friends of Tom and his sister Laura, reportedly told him: "You've been a bloody fool. Pull yourself together."

Prince Charles: Furious

The BBC court correspondent Jennie Bond said that the prince was very angry but also concerned.

He summoned his sons to Highgrove for a frank face-to-face discussion on the sensitive issue of drugs.

There are reports that William has had a cigarette or two but there is no suggestion that the princes have been involved with drugs.

And Tom Parker Bowles has reassured Prince Charles that he has never taken drugs in front of the princes.

Fighting drugs is an issue close to Prince Charles' heart and he is patron of the drugs charity Phoenix House.

"He's been very frank in discussing these issues with the people in our services and they have responded in the same way," said Bill Puddicombe of Phoenix House. "So he's concerned about the individuals and the stories he's heard."

Anxious Camilla

According to royal sources, Tom does not have a serious problem and that it is just an occasional habit.

Prince William: Heart-to-heart chat with father

However his mother was believed to be deeply saddened by the reports four years after her son was cautioned for possession of cannabis and ecstasy.

Tom's father, Andrew, visited ex-wife Mrs Parker Bowles to discuss the matter.

Their son was said to be feeling "contrite" and promised his parents he would steer clear of drugs in the future.

In Cannes, Tom's employers, PR firm Dennis Davidson Associates, assured reporters his job was safe and said that he had always behaved in a professional manner.

'Coke den'

Sunday newpapers had reported that Tom was "high on coke" while at the Cannes Film Festival.

Front-page news on the tabloids

The News of the World said Mr Parker Bowles told a friend he "did a line with someone I found last night". Undercover journalists reportedly overheard the conversation at a pavement cafe.