The Duchess of Cambridge is hosting the festive event at Westminster Abbey. The carol concert is set to be broadcast on ITV on Christmas Eve at 7.30pm.
Kate wore a festive red dress and heels for the carol service, while William was smart in a suit and tie matching his wife.
In a touching nod to Her Majesty, the Duchess wore a pair of earrings owned by the Queen.
The Together At Christmas concert reflects on the challenges of the Covid pandemic and will pay tribute to the people and organisations who have supported communities through the crisis.
Carols will be sung by the Westminster Abbey choir as part of the festive event.
There will also be performances by singers Ellie Goulding and Leona Lewis.
The Duke of Cambridge will give a reading, as will British Paralympian and junior doctor Kim Daybell, and presenter Kate Garraway, whose husband Derek Draper spent a year in hospital after contracting coronavirus.
And there will be a performance of To The Day, a composition created for the occasion by poet and writer Lemn Sissay.
Kate will feature in the introduction to the special Christmas Eve programme Royal Carols: Together At Christmas, as will a performance by singer Tom Walker of his Christmas single For Those Who Can't Be Here.
The service will both celebrate the heroes who stepped up to help others during the pandemic and reflect on the unprecedented difficulties the nation has faced over the last 18 months.
Kensington Palace said: "Unsung heroes from across the length and breadth of the nation have been invited to attend the service in recognition of their tireless efforts and selfless acts of kindness despite incredibly difficult circumstances."
Among the guests will be those involved in community networks, charitable organisations and the patronages of the Queen, Prince Charles and Camilla, and the Cambridges.
People who William and Kate met and spent time with during recent engagements have also been invited, as have armed forces personnel involved in Operation Pitting - the UK's evacuation of civilians from Afghanistan after the Taliban takeover.
Guests experiencing isolation, such as those with limited social connections, the recently bereaved or those without their own homes, will also be in the church.
The concert is being aired on ITV after a last-minute switch from the BBC following a row over a documentary last month.
BBC Two's The Princes and the Press, presented by self-avowed republican Amol Rajan, looked at the relationship between William, Prince Harry and the media.
In a rare joint statement, Buckingham Palace, Clarence House and Kensington Palace hit out at the two-part documentary.
The three royal households criticised the programme for giving credibility to "overblown and unfounded claims".
The controversial documentary prompted the royals to snub the BBC and move the broadcast of the carol service to ITV.
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