Britain's Princess of Wales has spoken of her relief at completing her course of chemotherapy, saying in a video message that she is doing what she can to remain "cancer free".

In an emotional message, accompanied by footage of Catherine and her family, she said her illness had given her a "new perspective on everything" and a focus on the "simple yet important things in life".

The intimate video voiced by the princess, 42, shows her with Prince William, Prince George, Princess Charlotte and Prince Louis spending time together in Norfolk.  

It comes after a shock cancer diagnosis earlier this year. 

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

She said she was relieved that her course of preventive chemotherapy was over after an "incredibly tough" and "scary" nine months for her family. 

Acknowledging that the "cancer journey" brought patients "face-to-face with your own vulnerabilities" she said "life as you know it can change in an instant".

She will begin a gradual return to a light programme of royal engagements but said she would be focusing on doing what she could to "stay cancer free". 

Britain's Catherine, Princess of Wales, smiling with Britain's Prince William, Prince of Wales, and their children Britain's Prince George of Wales, Britain's Princess Charlotte of Wales and Britain's Prince Louis of Wales. Photo: AFP

The princess added that her "path to healing and full recovery is long and I must continue to take each day as it comes". 

The video, shot in August, shows the family as they have never been seen in the public before with the prince and princess filmed kissing and the family embracing on the beach.

It has been a difficult time for the royal family with both the princess and her father-in-law, Britain's King Charles, diagnosed with cancer early this year. 

In March, the princess confirmed that pre-cancerous cells had been found following abdominal surgery and that she would have to undergo a course of preventative chemotherapy. 

"William and I are so grateful for the support we have received and have drawn great strength from all those who are helping us at this time," she said. 

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

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

The princess, who has not disclosed the nature of the cancer, is expected to take part in a number of events including the annual Remembrance service at the Cenotaph in November.