In a statement, she said her children had been brought up to be anti-drugs, and she was disappointed that her son, who is 28th in line to the throne, had experimented in this way.

The 20-year-old great-grandson of King George V revealed he had taken the drug at a party in London.

He has assured his mother that it would not happen again.

Lord Windsor, who is studying classics at Magdalen College, Oxford, is a cousin to Queen Elizabeth II.

'Not blaming anyone'

He is currently in New York on work experience with a finance company.

The Sunday Times newspaper quoted Lord Windsor as saying: "I admit it is true. It is very difficult to avoid getting into this sort of thing when you move in these circles, but I don't blame anyone else for the incident."

He said he had now rejected that type of lifestyle and was going to commit himself to his studies.

The incident follows the publication of a report by the British Crime Survey that cocaine use has soared among young people in England and Wales. This has sparked fears that its links with a celebrity lifestyle will make it the new fashion drug.

Camilla Parker Bowles' son Tom Parker Bowles and "It Girl" Tara Palmer-Tomkinson are among those who have recently admitted using the drug.