Catherine, Princess of Wales, on Saturday makes a tentative return to public life for the first time since being diagnosed with cancer, attending the Trooping the Colour military parade in central London.

Kate, as she is widely known, has not been seen at a public engagement since a Christmas Day service in December last year, and revealed in March that she was receiving chemotherapy.

With the 42-year-old princess one of the world's most-photographed women, her protracted absence has fuelled speculation, particularly online, about her health and whereabouts.

In a lengthy social media post on Friday evening, she said she was "making good progress" with her treatment, which she added is set to last for several more months.

"I'm looking forward to attending The King's Birthday Parade this weekend with my family and hope to join a few public engagements over the summer, but equally knowing I am not out of the woods yet," the princess said.

Kate's emotional announcement that she had cancer came just weeks after it was disclosed that her father-in-law King Charles III had also been diagnosed with the condition.

Neither has revealed what type of cancer they have.

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

His first engagement was meeting staff and patients at a London cancer treatment centre. Earlier this month, he attended commemoration events in northern France for the 80th anniversary of D-Day.

But unlike previous years, when he inspected troops on horseback at Trooping the Colour, Charles will do so this year from a carriage.

His elder son and heir William, 41, will be on horseback.

- Balcony -

Royal officials will be keen to manage expectations about Kate's gradual return to the public eye, and have maintained that her appearances will depend on her treatment and recovery.

Kate explained in her statement that she has had "good days and bad days", and was "taking each day as it comes".

She is expected to travel with Prince George, aged 10, Princess Charlotte, nine, and six-year-old Prince Louis in a state carriage down The Mall from Buckingham Palace to Horse Guards Parade.

They will watch the parade from a building before returning to the palace for a balcony appearance.

Trooping the Colour marks the British sovereign's official birthday and is a minutely choreographed military tradition dating back more than two centuries.

It starts at Buckingham Palace and moves down The Mall to Horse Guards Parade, where Charles will receive a royal salute before inspecting soldiers.

Charles was actually born in November but the second birthday tradition dates back to King George II in 1748, who wanted to have a celebration in better weather as his own birthday was in October.

The ceremony has its origins in the preparations for war, where all regimental flags -- or colours -- were shown to the soldiers so that they would recognise them in the confusion of battle.

This year's event will include three of five military horses that bolted through the streets of central London in April after being spooked by the noise of building construction.

London's Metropolitan Police said it would mount a "significant" security operation, and had been liaising with anti-monarchy group Republic, which plans to protest at the event.

The force said it had banned "amplified sound" in and around the parade route on public safety grounds and to avoid disruption to the mounted regiments taking part.