By ALANAH ERIKSEN
Letting her hair down: The Duchess of Cambridge arrives at the Imperial War Museum in London for the Sun Military Awards in a strapless black velvet dress
Just hours before, she joined her husband for a Christmas lunch at Buckingham Palace with the Queen.
But it was just the young royal set at the Sun Military Awards tonight.
The Duchess of Cambridge was the belle of the ball in a strapless black velvet dress as she walked the red carpet with her husband Prince William, who looked dapper in black tie.
Dressed to impress: The dress was made by a designer for Alexander McQueen and featured frill trim around the bust
Prince Harry was two steps behind his older brother at London's Imperial War Museum, also donning a suit.
Kate's black Alexander McQueen gown - the same designer as her wedding dress - had frill trim around the bust and a fishtail-style skirt.
It was so long on the five-foot, 10-inch Duchess, that she had to keep hitching it up to keep it from scraping on the ground.
As slim as ever: The tiny Duchess' waist was on show in the dress which had a fishtail-style skirt
Classic: Kate, whose shiny brunette locks were styled in loose curls, wore a chunky silver necklace and matching earrings and a bracelet which she received as a wedding gift
She held a matching black velvet clutch and donned black pumps.
The 29-year-old accessorized with jewellery she received as a wedding present - a chunky diamond necklace and a matching bracelet and earrings.
Her shiny brunette locks had been styled into loose curls with a side parting for the night.
Kate and her men: The Duchess arrives on the red carpet with her husband, Prince William and Prince Harry
The best sister-in-law: Kate shares a laugh with Harry on the red carpet as the cameras snap away
Leading the pack: Kate walks two steps ahead of the princes on the red carpet
Kate was still sporting the 'Scousebrow' - thick eyebrows made famous by the Liverpudlian show, Desperate Scousewives.
And she donned her trademark thick black eyeliner around her green eyes.
A lashing of light pink lipstick and her look was complete.
The royal arrival: The young princes looked dapper in black tie at the event
Gorgeous couple: Kate continued to hitch up her dress so as not to let it scrape on the ground
Big night ahead: The couple are escorted to their seat in the museum to watch the award show
Among the celebrity guests was David Beckham and his two eldest boys Brooklyn,12, and Romeo, nine.
The trio - looking dapper in suits - looked as though they had caught up on sleep after flying in from Los Angeles this morning with the rest of the family.
The Beckhams have returned to London for a British Christmas at 'Beckingham Palace' in Hertfordshire which Victoria is said to have decorated.
Me and my boys: David Beckham attended with his eldest sons Brooklyn, 12, and Romeo, nine
Doting daddy: Football ace David leads the well-behaved boys up the red carpet for the award show
Leading the night: The Prime Minister David Cameron and his wife Samantha
Two different worlds: Strictly Come Dancing judge Alesha Dixon looked gorgeous in this strapless beige dress, while reality star Amy Childs wore a more Essex-style purple gown
Coronation Street star Brooke Vincent looked glamorous in a floor-length, long-sleeved black gown.
It featured a scooped cowl-neck in green sequins.
It is the same dress that Strictly Come Dancing runner-up Chelsee Healey wore for the show's after-party on Saturday.
David Cameron launched the awards today with a message to the heroes: 'The whole country wants to say thank you.'
Glitz and glam: Future Dancing On Ice host Christine Bleakley wore this eye-popping black T-shirt dress while Louise Redknapp wore this long-sleeved silver number
Glam: Coronation Street star Sacha Parkinson showed some leg while her co-star Brooke Vincent sizzled in this black and green number
Different strokes: Coronation Street sat Catherine Tyldesley and Katie Price's ex-husband, Australian singer Peter Andre
Stylish: Strictly star Ola Jordan looked glamourous in this grey number while singer Myleene Klass chose an odd dress with dandelions on it
Speaking at a Downing Street reception, the Prime Minister and Deputy PM Nick Clegg joined forces to heap praise on the 'Herculean bravery' of the phenomenal men and women across the services.
More than 80 nominees and their partners packed into the historic No10 state room where the politicians mingled with the guests.
McSuited: Strictly Come Dancing winner Harry Judd and his McFly bandmate, I'm a Celebrity winner Dougie Poynter and comedian David Walliams and his mother Kathleen
Suited up: Gordon Ramsay also attended with his wife Tana along with Top Gear host Jeremy Clarkson and his wife Frances Cain