William and Kate, Prince and Princess of Wales celebrate13th wedding anniversary today

Kate, who is undergoing chemotherapy for an undisclosed cancer, married future king William in 2011 after eight years of dating.

The pair are facing their most difficult challenge so far amid the princess's diagnosis, with Kate appealing for time, space and privacy as she continues her treatment.

She praised her husband for his support during “an incredibly tough couple of months for our entire family” in an emotional message to the nation last month announcing her health news.

“Having William by my side is a great source of comfort and reassurance,” she said.

Kate, 42, told how her diagnosis had come as a huge shock and that she and William “have been doing everything we can to process and manage this privately for the sake of our young family”.

The prince returned to work following the Easter holidays when the Waleses spent quality time with their three children Prince George, Princess Charlotte and Prince Louis.

Louis celebrated his sixth birthday on Tuesday and Charlotte turns nine on Thursday.

William and Kate released an unedited new photo of Louis straight to social media to mark the youngster's big day to thank well-wishers and avoid the uproar over the princess's digitally altered Mother's Day photo.

The prince is set to carry out engagements in the North East this week, visiting an Earthshot Prize finalist firm which makes low-carbon construction materials in Seaham, and opening James's Place – a centre which offers free, life-saving treatment to suicidal men – in Newcastle tomorrow.

Life partners for more than 20 years, William and Kate met at the University of St Andrews in Fife, Scotland, and became friends first before embarking on a romance.

William said in their engagement interview: “Obviously we both have a very fun time together, both have a very good sense of humour about things.”

The princess called him a “loving boyfriend” who was “very supportive of me through the good times and also through the bad times”.

They wed in a grand fairytale ceremony in Westminster Abbey on April 29 2011, with the aisle of the gothic church lined with trees.

Thirteenth wedding anniversaries are traditionally marked with the gift of lace.