Kate Middleton
The Duchess of Cambridge is relaxing on the Caribbean island of Mustique (Reuters)

Prince William and Kate Middleton have flown of the Caribbean island of Mustique and are staying in a luxury £19,000-a-week villa.

The Duke and Duchess of Cambridge are treating themselves to a second honeymoon before their firstborn arrives in July.

A source told the Sun that the couple have been joined by Kate's parents, Michael and Carole Middleton, who are staying nearby on the island.

The Middletons were said to be happy that Kate and William could make the trip following Kate's bout of acute morning sickness, which resulted in a stay in hospital in December.

They said: "The duke and duchess have joined the Middletons for a winter break.

"Both William and Kate were hoping her condition would not stop her from making the eight-hour flight to Barbados and the connection to Mustique.

Mustique is a favourite spot for the Middleton family (Wiki Commons)

"A month ago such a journey would have been unthinkable for her, so this is a clear sign she has made a virtual recovery."

Mustique, a small private island which is part of St Vincent and the Grenadines in the West Indies, is popular with the rich.

The royals' villa comes complete with an infinity pool, a games room and views of the Caribbean Sea.

Their visit to the island appears to becoming an annual event. They were last there in January 2012.

Queen Elizabeth II and Prince Philip have visited Mustique on three occasions, and Jennifer Lopez, Mick Jagger and Amy Winehouse have all spent time there.