Princess of Wales beams as she joins sister Pippa Middleton and a host of stars at her annual Westminster Abbey carol concert - before George, Charlotte and Louis arrive tonight to see special rendition of Last Christmas played on John Lennon's piano

The Princess of Wales joined her sister Pippa Middleton at Westminster Abbey today ahead of her annual Christmas carol service as she met celebrities taking part.

Kate Middleton was not joined by her family who are expected to arrive later. She is set to be accompanied by her three children as well as her husband for the event. 

Among the group waiting for Kate in the Abbey's Lady Chapel were singers Beverley Knight and Adam Lambert, musician Jacob Collier and actor Jim Broadbent.

Kate's sister Pippa and husband James Matthews were also photographed arriving this afternoon, as were Lady Sarah Chatto and her husband Daniel Chatto.

The Princess will tonight unveil a very special festive gift to guests at the concert - a rendition of Last Christmas played on John Lennon's piano.

She has been loaned the iconic instrument - on which the former Beatle composed the song 'Imagine' – by the estate of the late George Michael.

The singer – who himself died tragically early, on Christmas Day 2016 - bought it in 2000 for a staggering £1.45million.

His famed Christmas hit will be performed by British Grammy award-winning singer/songwriter Jacob Collier in what will undoubtedly be the highlight of the royal service, now in its third year.

Royal Carols: Together at Christmas will be filmed tonight and broadcast on Christmas Eve on ITVI and ITVX.

Spearheaded by the princess and supported by The Royal Foundation, WLiliam and Kate's charitable organisation, it is designed to recognise those who have gone above and beyond to help others throughout the year.

The theme this year is Kate's 'early years' campaign, designed to highlight how important what happens to children between the ages of birth to five is in shaping their development as adults.

Among the 1,500 invited guests tonight will be parents, carers, early educators, midwives, health visitors and community volunteers.

All have either been nominated by Lord-Lieutenants across the nation or charities associated with the Royal Family, as well as people the princess knows personally through her work in the field with her Royal Foundation for Early Childhood Centre.

Some families with young children who have had a particularly challenging year have also been invited too after being nominated by charities the princess has associations with.

The service, combining traditional and modern elements, will see the Westminster Abbey choir perform alongside musicians including singers Freya Ridings and James Bey, with a special duet from Beverley Knight and Adam Lambert of The Christmas Song, made famous by Nat King Cole.

Readings will be delivered by Prince Wiliam, actor Jim Broadbent, presenter Emma Willis and actor Michael Ward.

Presenter Roman Kemp will introduce the special rendition of Last Christmas on Lennon's piano, talking about the role George Michael played in his own life – his mother, Shirlie Kemp, was one of his backing singers - and the story of the piano itself.

A source told the Mail that the instrument had been loaned by his estate who were keen to support this year's service. 

Lennon, who was killed by gunman Mark Chapman in December 1980, bought the piano in 1970 and had it delivered to studios at his home in Tittenhurst Park, Berkshire.

He composed and recorded Imagine on it the following year, and was filmed playing the song on it for the first time to wife Yoko Ono and the Plastic Ono band.

It went on to become the best-selling single of his solo career

George Michael, who subsequently composed his own music using it including the title track of his 2004 album Patience, said he bought the piano because it was 'not the type of thing that should be in storage somewhere or being protected, it should be seen by people'.

Tonight's service will be attended by the princess' family, friends and other members of the Royal Family.

Although he will not be present due to a diary clash, the King has donated Christmas trees from Windsor Great Park to decorate Westminster Abbey, which will be donated to the local community after the service.

The abbey will also be filled with sustainable, eco-friendly festive decorations.

Outside the princess has had a special post box installed, with guests encouraged to bring a card which will be distributed to children who might be struggling this year through charities including Child Bereavement UK, East Anglia Children's Hospice, Evelina London and the British Red Cross.

They will also be greeted by carols and Christmas songs by the Action for Children Young Carers Aloud Choir, made up of 12 young carers aged six upwards.

A royal aide said the princess – who will not speak on the night but will feature in a pre-filmed introduction to be broadcast on Christmas Eve – hoped the service would remind people of 'how precious' the early years of a child's life at a time when the world celebrates the birth of a newborn baby.

'It is a reminder of how unique this time is, and how precious those early years are, as we start to form our very first relationships, bond with those closest to us, and explore the world in which we live,' they said.

'It also signifies hope. As we look to the future, we hope our children will grow up in a happier, healthier, fairer, kinder and more tolerant world. A world based on love and compassion.

'This evening, Westminster Abbey will be filled with more than 1,500 inspiring people in appreciation of their selfless support as they work tirelessly to help the youngest members of our society to flourish.'

Royal Carols: Together at Christmas will air on December 24 at 7.45pm on ITV1 and ITVX