At least one member of staff tried to access Kate Middleton’s notes while she was a patient at the private hospital in central London in January, The Mirror reported.
The princess was admitted to the hospital for abdominal surgery on January 16.
Details of Kate’s condition have not been disclosed but Kensington Palace previously said it was not cancer-related and that the princess wished for her personal medical information to remain private.
The Sun newspaper shows the Princess of Wales with the Prince of Wales during a visit to a farm shop in Windsor (Jonathan Brady/PA)
The UK privacy and data protection watchdog said it had received a breach report.
An Information Commissioner’s Office (ICO) spokesperson said on Tuesday: “We can confirm that we have received a breach report and are assessing the information provided.”
Kensington Palace said: “This is a matter for The London Clinic.”
Kate’s absence from public life, after her surgery, has led to wild conspiracy theories on social media about her whereabouts and health.
Footage emerged of Kate out shopping with the Prince of Wales at the weekend at the Windsor Farm Shop close to their Adelaide cottage home in the grounds of Windsor Castle.
Nelson Silva, 40, who filmed Kate shopping, told The Sun he believes the conspiracy theorists are “delusional”.
He said: “I’m not so much shocked that these comments have continued, I’m just confused how exactly they can continue.
“This is a video clearly showing her and William. I saw them with my own eyes. It was a completely relaxed situation.”
The royal couple also spent Sunday morning watching Prince George, Princess Charlotte and Prince Louis taking part in a sporting event, according to The Sun.
The online speculation increased after irregularities were spotted in a Mother’s Day photograph of Kate and her children, which led to the princess admitting to “editing” the image and apologising for any “confusion” caused.
The future Queen of England has been photographed in public twice since her operation.
In a picture published on March 4, she was a front-seat passenger in a car driven by her mother, Carole Middleton, in the Windsor area.
She was also spotted sitting next to William in a chauffeur-driven car last Monday, when he attended the Commonwealth Day service in London and she was driven to a private appointment.
It was reported at the weekend that Kate may speak about her health during public engagements which are not expected to resume until after Easter.
The Metropolitan Police and The London Clinic have been approached for comment.
