Earl Spencer has apologised to Piers Morgan over a factual error in his new book about Diana, Princess of Wales.

In his memoir, Earl Spencer said Morgan was editor of the Daily Mirror when the Mirror Group published "highly intrusive photographs of Princess Diana at the gym".

Earl Spencer, Diana's brother, admitted "his assumption was incorrect" and "apologises to Mr Morgan for this error", a statement said.

The BBC understands Penguin, the publishers of Swan Song: Diana, My Sister, will be amending this one point in future editions of the book.

Morgan has acknowledged Earl Spencer's apology, but doubled down on his threat of legal action.

In a post on social media, Morgan said: "I am pursuing legal action against him and his publisher Penguin to have the lies removed, and damages paid to the King's Trust."

The photographs of Princess Diana mentioned in relation to Morgan in Earl Spencer's book were published by the Sunday Mirror on 7 November 1993.

At the time, Morgan was working at The Sun and did not join the Mirror as its editor until 1995.