Kate, Britain's Princess of Wales, said on Friday she was undergoing preventative chemotherapy after tests taken following major abdominal surgery in January revealed cancer had been present.

Kate, 42, the wife of heir-to-the-throne Prince William, spent two weeks in hospital in January after having what her office said at the time was successful, planned surgery for a non-cancerous but unspecified condition.

However, in a video message, Kate said subsequent tests had revealed cancer had been found, but she said she was well and getting stronger.

"My medical team therefore advised that I should undergo a course of preventative chemotherapy and I am now in the early stages of that treatment," Kate said in the video which was filmed on Wednesday.

"This of course came as a huge shock, and William and I have been doing everything we can to process and manage this privately for the sake of our young family."

King Charles revealed in February that he too was to have treatment for cancer, meaning he has had to postpone his public royal duties.

Her office, Kensington Palace, said it would give no further details about the type of cancer. It said she was on a recovery pathway and the preventative chemotherapy had begun in February.

After her operation, the palace said Kate would not return to official duties until after Easter, but her absence from public life has provoked intense speculation on social media.

She and William had wanted privacy about the cancer until their children, Prince George, 10, Princess Charlotte, 8, and Prince Louis, 5, began their school holidays which started on Friday.

"It has taken me time to recover from major surgery in order to start my treatment. But, most importantly, it has taken us time to explain everything to George, Charlotte and Louis in a way that is appropriate for them, and to reassure them that I am going to be ok," she said.

"As I have said to them; I am well and getting stronger every day by focusing on the things that will help me heal; in my mind, body and spirits."

Kate has not appeared at a public event since she joined other members of the royal family for a church service on Christmas Day.