Britain's Prince William leaves after visiting his wife Catherine, Duchess of Cambridge at the King Edward VII hospital in London December 5, 2012. REUTERS/Neil Hall

London - The British hospital treating Prince William's pregnant wife Catherine admitted on Wednesday that it released her private medical details to hoax callers from an Australian radio station.

Presenters from Sydney's 2Day FM station, posing as Queen Elizabeth II and William's father Prince Charles, got through to a nurse at London's private King Edward VII's Hospital, where Kate is being treated for acute morning sickness.

The hospital confirmed that a nurse discussed Kate's condition and said it “deeply regrets” the incident, which happened in the early hours of Tuesday, while the radio station has apologised.

The hoax is deeply embarrassing for the hospital, which has also treated the queen, her husband Prince Philip and Charles' wife Camilla over the years. The queen is its patron.

“This was a foolish prank call that we all deplore,” said the hospital's chief executive John Lofthouse.

“We take patient confidentiality extremely seriously and we are now reviewing our telephone protocols.”

2Day FM presenter Mel Greig, impersonating the 86-year-old monarch, dialled the hospital and asked the operator: “Could I please speak to Kate, please, my grand-daughter.”

The operator replied: “Oh yes, just hold on, Ma'am.”

The call was put on hold and Greig's co-presenter Michael Christian asked incredulously: “Are they putting us through? If this has worked, it's the easiest prank call we've ever made. Your accent sucked, by the way.”

Greig was put through to a ward where a nurse told her that Kate was “sleeping at the moment and she has had an uneventful night”.

The nurse added: “She's been given some fluids to rehydrate her because she was quite dehydrated when she came in. But she's stable at the moment.”

Greig replied, “Oh well, I'll just feed my little corgis, then,” while two of her colleagues made barking noises in an impression of the queen's corgi dogs.

“So, when is a good time to come and visit her?” Greig went on. “Because I'm the queen, so I need a lift down there.”

She then asked Christian, posing as 64-year-old Charles, when he could take her to the hospital.

The nurse suggested any time after 9:00 am, after Kate had “freshened up”.

“She hasn't had any retching with me since I've been on duty and she has been sleeping on and off,” the nurse added.

2Day FM said it “sincerely apologises” for any inconvenience caused by the prank.

The station tweeted: “The radio segment was done with the best intentions and we wish Kate and her family all the best.”

A spokesman from William and Kate's office at St James's Palace declined to comment.

Meanwhile William, second in line to the throne behind Charles, was back at his wife's bedside on for a third day, while Kate's younger siblings Pippa and James also paid her a visit.

Casually dressed in beige corduroy trousers and a blue sweater, William spent five hours at the hospital, smiling at the international media camped outside the building as he left.

The Church of England has released a prayer for the couple that its 16 000 churches can use in Sunday services.

Kate is suffering from hyperemesis gravidarum, a severe form of morning sickness that affects about one in 200 pregnant women.

News of her pregnancy has ended the feverish speculation about a new royal heir that began immediately after the couple's lavish wedding in April 2011.

The baby will be third in line to the throne.

For the first time, if the child is a girl she cannot be leap-frogged in the line of succession by a younger brother, following last year's historic agreement among Commonwealth realms to end the practice of male primogeniture.

It is not the first time that the royal family have been targeted by hoax callers.

In 1995 Canadian DJ Pierre Brassard, posing as then Canadian prime minister Jean Chretien, was put through to the queen.

The pair spoke for around 15 minutes and Brassard even managed to elicit a promise that the queen would try to influence Quebec's referendum on proposals to break away from Canada. - Sapa-AFP