Kate planned carol service with Lady Gabriella Windsor

The Princess of Wales asked Lady Gabriella to be involved in the summer with Catherine understood to have been incredibly grateful for her contribution.

Lady Gabriella is the daughter of Prince and Princess Michael of Kent - with Prince Michael, aged 81, a grandson of King George V and first cousin of the late Queen Elizabeth II.

Lady Gabriella's husband, financier Thomas Kingston, died from a head injury at his parents' home in the Cotswolds. In October, a coroner concluded that he took his own life at the age of 45.

In a tribute shared at the time of his death, Lady Gabriella and his family called Mr Kingston "an exceptional man who lit up the lives of all who knew him".

The Christmas message at this year's service was about promoting "love, not fear".

"Love is the light that can shine bright, even in our darkest times," the princess wrote in a letter to guests.

It marked the end of what has been a difficult year for her and her family with both Catherine and King Charles undergoing cancer treatment.

The Prince and Princess of Wales were joined by their children Prince George, Princess Charlotte and Prince Louis at the service.

The 1,600-strong congregation all held candles during the service that will be broadcast by ITV on Christmas Eve.

The world-renowned Abbey Choir performed a number of carols and the event featured performances by Paloma Faith, Gregory Porter, Olivia Dean, and singer-songwriter JP Morgan who performed alongside the Soul Sanctuary Choir.

Readings were given by actor Richard E Grant, Olympic swimming gold medallist Adam Peaty and Downton Abbey star Michelle Dockery.

It will feature three films about people and organisations that have inspired and comforted others in times of need.

In a pre-recorded opening address, Catherine will read extracts from a letter she wrote about love and empathy that was included in the event's order of service.

She will say: "The Christmas story encourages us to consider the experiences and feelings of others.

"It also reflects our own vulnerabilities and reminds us of the importance of giving and receiving empathy, as well as just how much we need each other in spite of our differences.

"Above all else, it encourages us to turn to love, not fear. The love that we show ourselves and the love we show others. Love that listens with empathy, love that is kind and understanding, love that is forgiving, and love that brings joy and hope."