Kate Middleton to make first public appearance since December as ...

The Princess of Wales says she will "join a few public engagements over summer", three months after it was revealed she was receiving preventative chemotherapy for cancer.

In a statement, Princess Catherine said she would attend the Trooping the Colour ceremony in London on Saturday — one of the largest events on the royal calendar.

However, she also said her treatment was "ongoing, and will be for a few more months".

"On the days I feel well enough, it is a joy to engage with school life, spend personal time on the things that give me energy and positivity, as well as starting to do a little work from home," she said in the statement, released 6pm Friday, local time (3am Saturday, AEST).

"I am learning how to be patient, especially with uncertainty.

"Taking each day as it comes, listening to my body, and allowing myself to take this much needed time to heal."

A new photo of the princess was also released to mark the announcement.

This picture of Princess Catherine was released to accompany the royal announcement on Friday.(Supplied: Matt Porteous)

Kensington Palace announced in January Catherine would be stepping away from royal duties while she recovered from abdominal surgery.

However, in March, the princess released a video message announcing she was receiving preventative chemotherapy for cancer.

The specifics of her condition have not been made public.

While the Princess of Wales is attending Trooping the Colour, it's understood this does not mean she is returning to full public duties.

It's understood she will travel down the Mall in a state carriage with her children and watch the parade.

She is also expected to appear on the balcony at Buckingham Palace with other members of the royal family later in the day.

"I have been blown away by all the kind messages of support and encouragement over the last couple of months," the princess said in her statement.

"It really has made the world of difference to William and me and has helped us both through some of the harder times.

"I am making good progress, but as anyone going through chemotherapy will know, there are good days and bad days.

"On those bad days you feel weak, tired and you have to give in to your body resting.

"But on the good days, you feel stronger, you want to make the most of feeling well."

While the Princess of Wales said she would like to attend more events in the coming months, none have been announced.

She said she was "not out of the woods yet".

It's understood the princess has started to take some meetings related to her royal duties, and projects.

A Buckingham Palace spokesman said: "His Majesty is delighted that The Princess is able to attend tomorrow's events, and is much looking forward to all elements of the day."

In January, it was announced King Charles III had undergone surgery to treat an enlarged prostate.

It was subsequently revealed he had been diagnosed with a form of cancer, although specifics are not known.

He stepped away from public duties for about 12 weeks, but has since returned to them.