The Middletons are believed to be on the isle of Mustique, without William

By Simon Perry
Updated January 20, 2012 10:45 AM
Getty; Inset: PA/Landov

Rainy and 48 degrees in London on Friday. No matter: The Duchess of Cambridge – with parents Michael and Carole, and sister Pippa – is off on a sunny vacation, PEOPLE has confirmed.

Passengers on a British Airways flight from London’s Gatwick Airport to St. Lucia in the Caribbean were surprised to see the family in the first class section on Wednesday, a photographer blogged. Their presumed final destination: one of their regular holiday haunts, the private isle of Mustique.

It is believed that Prince William is not with them, as the search and rescue helicopter pilot is spending this week at his RAF base in Anglesey, Wales, after hunting last weekend on an estate in Spain with brother Prince Harry.

Mustique, where Kate and William have been frequent visitors, is a well-known, pricey haunt for such celebrities as Mick Jagger and Queen Elizabeth’s late sister, Princess Margaret. It was even inspected as a possible honeymoon destination for William and Kate.

The trip to the sun comes just days after Kate’s 30th birthday on Jan. 9 and before Carole’s 57th at the end of the month.

The Palace had no comment on Kate’s vacation.