WATCH: Catherine, Princess of Wales says she has completed chemotherapy

The heartfelt announcement will provide welcome relief for Britain's royal family after a troubled year in which head of state King Charles III also revealed he was battling cancer.

He was seen on Saturday with his wife Queen Camilla and other members of the royal family at the Braemar Gathering Highland Games, and attended a church service to mark the second anniversary of his accession to the throne.

The couple are also set to undertake a state visit to Australia and Samoa next month, albeit with a slimmed-down timetable as he recovers.

"I cannot tell you what a relief it is to have finally completed my chemotherapy treatment," Kate, as she is often called, said in a message on X and Instagram.

The princess, 42, said the last nine months had been "incredibly tough" for the family -- her husband Prince William and their children Prince George, 11, Princess Charlotte, nine, and Prince Louis, six.

"Life as you know it can change in an instant and we have had to find a way to navigate the stormy waters and road unknown," she said.

Catherine, a future queen seen as a key figure to maintain the royals' position in a changing Britain, added that "doing what I can to stay cancer free is now my focus".

"Although I have finished chemotherapy, my path to healing and full recovery is long and I must continue to take each day as it comes," she said.

"I am however looking forward to being back at work and undertaking a few more public engagements in the coming months when I can."

Catherine announced in March that she was undergoing a course of "preventative chemotherapy" for an undisclosed cancer.

- Solidarity -

The announcement came just weeks after it was disclosed that her father-in-law had also been diagnosed with cancer after undergoing treatment for a benign prostate.

Charles, 75, was given the green light to resume public duties in April after doctors said they were "very encouraged" by his progress.

The video message shows Catherine and William looking relaxed and happy with their children in the Norfolk countryside and at the beach in eastern England where they have a home.

The couple are seen being openly affectionate to one another with William at one point planting a kiss on Catherine's cheek.

In other scenes, the family is seen walking through woodland, having a picnic and playing cards.

The video was filmed last month, their Kensington Palace office said.

Catherine added that the months since her diagnosis had "above all reminded William and me to reflect and be grateful for the simple yet important things in life, which so many of us often take for granted. Of simply loving and being loved."

"Everyone's kindness, empathy and compassion has been truly humbling," she said.

And she expressed solidarity with fellow cancer sufferers.

"To all those who are continuing their own cancer journey -- I remain with you, side by side, hand in hand. Out of darkness, can come light, so let that light shine bright," she said.

Catherine has appeared in public on only a handful occasions this year.

- Tentative return -

When she announced her cancer diagnosis in a video on March 22 she had not been seen in public since Christmas.

The disclosure came after royal officials said in January that Catherine would spend two weeks in hospital for abdominal surgery.

That announcement came shortly before a separate statement from Buckingham Palace that Charles was to have surgery.

He later spent three days at the same private hospital in central London. Officials then said in early February that tests had identified "a form of cancer", without giving further details.

Catherine tentatively returned to UK public life in June, attending a military parade in London to officially mark Charles's birthday.

In July, she attended the Wimbledon men's final at which she received cheers as she walked onto court to present the trophy to Carlos Alcaraz.