Britain's Duchess of Cambridge is set to be honoured by Queen Elizabeth.

Catherine, Duchess of Cambridge

Catherine, Duchess of Cambridge

The queen will celebrate becoming the longest-reigning monarch this month by bestowing the Royal Family Order of Queen Elizabeth II on her grandson Prince William's wife.

The order is the highest honour that a female member of the royal family can receive.

A source told the Daily Mail newspaper: "It is an honour reserved for first division female royals."

It is described as a "diamond- encrusted brooch featuring an ivory plaque of the queen wearing the Order of the Garter and bearing the Royal cypher".

The news comes after it was revealed that Catherine will accompany her husband on his Caribbean Royal Tour next year.

The couple will be joined by their children, Prince George, two, and three-month-old Princess Charlotte when they visit some of the 16 Commonwealth countries where Queen Elizabeth is Head of State.

The news comes amid speculation that the 33-year-old princess made a conscious decision to spend more time at home with her family - since she has been notably absent from William's side at a number of social engagements recently - following the birth Charlotte in May.

And Catherine even rescheduled her annual Middleton family holiday to Mustique - which usually takes place in January - to accommodate the trip that is due to take place early next year.