The newspaper said that the Princes are planning a comedy play for their father's 50th birthday later this month.

The statement, issued by St James's Palace, says the boys are upset that their secret has been spoiled.

It added: "The Prince of Wales is sad that the newspaper involved didn't handle the information it received with greater common sense and courtesy."

The incident follows careful discussions earlier this month between The Sun newspaper and the Palace over the revelation that Prince William had met his father's companion, Camilla Parker Bowles.

The Prince and his father were consulted before that story was published.

Emma Thompson to star

The Sunday Mirror said William, 16, and Harry, 13, will perform alongside the Oscar-winning British actress Emma Thompson and comic actor Stephen Fry, who wrote the script.

It said 100 guests will pay £25 to attend at Prince Charles' Highgrove estate on 31 July, three-and-a-half months before his official birthday.

The idea for the play came from Prince William who then asked his former nanny, Tiggy Legge-Bourke, to help organise it.

A spokeswoman for Prince Charles said Princes William and Harry were "very disappointed" that news of the surprise had leaked out.

She said: "The party will go ahead, though it isn't going to be a surprise any more. I am not going to talk about the details because it isn't fair to the boys.