Glum Prince Harry next to Kate Middleton and Prince William as Meghan Markle flies home

Harry appeared less cheery than usual on the outing with the Duke and Duchess of Cambridge.

The Duchess of Cambridge embraced the festive season on a crisp Monday morning (19 December) alongside husband Prince William and a rather melancholy Prince Harry as the royal trio stepped out in North Kensington.

Kate Middeton, 34, arrived inimiatbly in style for the Christmas party hosted by youth support service, The Mix, to celebrate the volunteers and counsellors who support people through difficult times in their lives.

Wearing a red and green printed dress which retails at £720 by Vanessa Seward – a new label for the duchess – Middleton smiled from ear-to-ear as she styled the Christmassy number with black stilettos and a waist-cinching black belt. She wore her hair in her trademark Chelsea blow-dried style while sporting a simple yet glowy make-up look.

She was accompanied by a cheerful Prince William – who presented Michael Phelps with the Lifetime Achievement Award at BBC's Sports Personality of the Year last night (18 December) – and a glum Prince Harry after he bid farewell to his girlfriend Meghan Markle on Sunday afternoon.

Harry – who spent a romantic week with Markle before he joins other members of the Royal Family at the Queen's Sandringham House for Christmas – struggled to crack a smile during the morning engagement with his cheery royal counterparts.

The 32-year-old prince was no doubt feeling deflated after Markle jetted back to Canada after he personally dropped her off at Heathrow airport once her UK stay came to an end. The pair were spotted jovially buying a Christmas tree together and attending a play at London's West End.

prince harry — Prince Harry arrives to attend a Christmas party for volunteers at The Mix youth service Chris Jackson/Getty Images

Harry has also given an emotional interview for a new documentary where he speaks about the untimely death of his mother, Princess Diana, in 1997.

The royal, who was just 12 years old at the time of her death, opened up in ITV's latest documentary, Prince Harry In Africa – which airs on ITV at 9pm tonight. It marks the 10th anniversary of the Aids charity Sentebale which he set up in her memory.

"I never really dealt with what had happened," the 32-year-old royal said. "It was a lot of buried emotion. For a huge part of my life I didn't really want to think about it."