Two presenters pretending to be the Queen and Prince Charles managed to obtain intimate medical information about the expectant royal, who is suffering from servere morning sickness.
Believing them to be genuine, the nurse who answered the call at King Edward VII Hospital told them Kate had had "a comfortable night" and revealed private medical information.
A hospital spokesman said: "This call was transferred through to a ward and a short conversation was held with one of the nursing staff. King Edward VII's Hospital deeply regrets this incident."
John Lofthouse, the hospital's chief executive, said: "This was a foolish prank call that we all deplore."
In a statement, 2Day FM – the radio station behind the hoax - said they "sincerely apologise for any inconvenience caused by the inquiry... The radio segment was done with the best intentions, we're glad to hear Kate is doing well and we wish her all the best."
The presenters behind the prank, Mel Grieg and Michael Christian (above), said: "We were very surprised that our call was put through, we thought we'd be hung up on as soon as they heard our terrible accents. We're very sorry if we've caused any issues and we're glad to hear that Kate is doing well."
The call was made in the early hours of the morning, before Prince William returned for the third day in a row to visit his wife, who is feeling much better, royal officials said on Tuesday.
A St James's Palace spokesman said: "The Duchess of Cambridge is continuing to feel better. She and the Duke are immensely grateful for the good wishes they have received. She will remain in hospital at present."
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