Prime Minister Rishi Sunak was also informed ahead of the announcement, with the King being forced to postpone many royal duties while he is undergoing treatment.
Mr Sunak tweeted earlier this evening: “Wishing His Majesty a full and speedy recovery. I have no doubt he’ll be back to full strength in no time and I know the whole country will be wishing him well.”
Former PMs Liz Truss and Boris Johnson also shared their best wishes on social media, as Ms Truss wrote: “Sending every best wish to His Majesty The King and the Royal Family as he undergoes his treatment for cancer. He will be in our thoughts and prayers.
“God Save The King!”