Cheeky Prince Louis causes mischief at Kate's carol concert as he blows out Charlotte's candle

The three posted letters to other children before The 'Together At Christmas' Carol Service at Westminster Abbey.

After the service, Prince Louis blew out Charlotte's candle as their mother Kate laughed. He was pictured puffing his cheeks in an attempt to extinguish his older sister's candle, as she smiled.

WATCH NOW: The Royal Family attend the service at Westminster Abbey

Prince George, ten, was dressed in a smart suit, while Prince Louis, five, wore a navy button-up jacket, narrow-legged trousers and blue suede Chelsea boots.

Princess Charlotte, eight, was in a £180 double-breasted burgundy wool coat from Trotters.

The service, supported by The Royal Foundation, honoured those who work to support babies, young children and families in communities across the UK.

Kate, who wore a white coat over a monochrome outfit, had travelled to the Abbey an hour earlier to check on arrangements.

Inside the Lady Chapel, she met the performers and told American singer Adam Lambert that she had been a 'big fan' of his performance at the Queen's Platinum Jubilee last year.

She also told how Charlotte was learning the piano and had watched British singer-songwriter Jacob Collier

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Collier's performance of the Wham! song Last Christmas on John Lennon's old piano was the highlight of the night.

The Princess herself unveiled a very special festive gift to guests at the concert by playing a rendition of Last Christmas played on John Lennon's piano.

Also present was Kate's sister Pippa, the Duchess of Edinburgh and Princesses Beatrice and Eugenie. Pippa was wearing a chic £450 tweed suit from Salvoni as she arrived at the Abbey accompanied by her husband James Matthews.

Beatrice's seven-year-old stepson, Christopher Wolfe 'Wolfie' Mapelli Mozzi, wore a beige jacket with green trim and bronze buttons as he joined his parents at the event.

Sophie, 58, wore a blue Suzannah London wool coat, layered over a figure-skimming maxi dress with a pair of £1,300 suede knee-high boots from Italian luxury shoe brand Gianvito Ross.

Princess Beatrice wore a £895 green wool tartan dress, which is part of Beulah London’s Winter 2023 collection.

Whereas her sister Eugenie wore a glamorous black ensemble with a £495 red leather handbag from designer Aspinal of London.

The event at Westminster Abbey will be broadcast on Christmas Eve on ITV1 and ITVX.