A formal complaint of harassment has been made against a newspaper by Prince William's girlfriend, Kate Middleton.
Kate Middleton posed for pictures at the Cheltenham festival
The Press Complaints Commission (PCC) said it was contacted following the publication of a photograph in Thursday's Daily Mirror.
The complaint alleges the picture breaches the PCC's rules as it was obtained "as a result of harassment".
In a statement, the Mirror said it would "provide the necessary thoughts and comments to the PCC".
The Mirror's photograph showed Miss Middleton walking to work, holding a cup and her car keys.
The PCC said it was currently investigating the complaint, and declined to comment further.
Miss Middleton had previously complained about the press intruding on her privacy.
In 2005, her lawyers contacted newspaper editors to ask the press to leave her and her family alone.
They claimed photographers had followed her almost every day and night since she left university.
In January, police had to escort the prince and Miss Middleton after they were surrounded by photographers while leaving a nightclub.
Shortly after that incident, News International - which publishes The Sun, News of the World, The Times and Sunday Times - introduced a self-imposed ban on using paparazzi pictures of Miss Middleton.
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In a statement, the PCC said: "The Press Complaints Commission has today received a formal complaint from Kate Middleton that a photograph of her in today's Daily Mirror was obtained as a result of harassment in breach of Clause 4 (Harassment) of the Code of Practice."
The clause states that "journalists must not engage in intimidation, harassment or persistent pursuit.
"They must not persist in questioning, telephoning, pursuing or photographing individuals once asked to desist; nor remain on their property when asked to leave and must not follow them.
"Editors must ensure these principles are observed by those working for them and take care not to use non-compliant material from other sources."