The Daily Mail is facing a substantial claim for damages after publishing pictures without copyright permission of Princes Harry and William revelling at a nightclub.
The pictures first appeared in The Sun on Tuesday, but were then reprinted in the Mail on Wednesday, headlined "Harry's fury over nightclub snaps".
The owner of the photographs, Natalie Pinkham, has taken no legal action against The Sun, but revealed on Wednesday night she is to sue the Mail for "flagrant" breach of copyright.
According to leading picture agency Rex Features, a UK licence for the pictures could have fetched up to £250,000 if originally sold on the open market.
If successfully sued for copyright, the Mail could be forced to pay double the pictures' theoretical price.
The Sun originally claimed the photos were taken this summer — so would have been of great interest to Harry's current girlfriend Chelsy Davy.
But on Wednesday the paper admitted the pictures in fact dated from autumn 2003, and were in Purple Nightclub and not Boujis as first stated.
The Sun also admitted that the photos were published without copyright permission.
Media lawyer Mark Stephens said: "Newspapers often take a calculated risk that they will have to pay damages, but that the economic benefits of publishing are worthwhile.
"The copyright damages are normally what the cost of a licence to publish the pictures would have been if they had been bought through Rex or someone like that.
"The damages can be doubled if a judge rules that the breach was deliberate or flagrant."
In a page-six Sun apology on Wednesday, it said: "The Sun published the photos in good faith, but we apologise to Ms Natalie Pinkham, a close friend of both Prince William and Prince Harry, for publishing them without permission, and for any embarrassment or offence their publication has caused.
"We will be making a donation to a charity of Ms Pinkham's choice."
The small apology appeared on a lefthand news-in-brief slot on page six.
A spokesperson for Clarence House said they would be taking no further action over the mistake — and will not be involving the Press Complaints Commission.
They said: "We are very pleased that they were able to correct it so quickly."