A royal specialist has outlined a forthcoming "key milestone" in Kate Middleton's medical journey. Kate revealed her cancer diagnosis in March and is now receiving treatment. The Princess of Wales will not take part in her customary role during the Colonel's Review, Kensington Palace informed the public last month.

Former ITN royal editor Tim Ewart said that Trooping the Colour will be a "crucial" time in Kate's rehabilitation. "We anticipate seeing her on the balcony of Buckingham Palace, which would be her first public appearance since the statement. People continue to be quite worried about her health," Ewart said to Sky News Australia.

Prince William hasn't provided many updates on Kate's health, Ewart noted. "We still don't know what kind of cancer Kate has or what treatment she is receiving, and there haven't been any formal medical bulletins," he said.

Being a Colonel in one of the participating regiments, Kate was expected to attend the Colonel's Review, a rehearsal held a week before the main parade commemorating the monarch's birthday. "She ought to have been there as one of the regiments involved is hers," Ewart said.

Irish Guards Colonel Kate wrote the regiment's commander a letter expressing her regrets. I'm sorry I can't take the salute at this year's Colonel's Review; being your Colonel is still a wonderful privilege. "I hope to represent you all again very soon; please pass my apologies to the whole Regiment," she wrote.