King Charles pictured greeting Rod Stewart - and is he drinking his favourite tipple?

King Charles was pictured having a drink with Rod Stewart at today's Foundation event.

The monarch, who is currently receiving treatment for an undisclosed form of cancer, made an appearance at Tuesday's event.

The event, which is set to become an annual event for The King's Foundation, aims to showcase the work of students, teachers, alumni and partners who have contributed to the organisation's charitable efforts.

Although what the King is drinking as he cheerfully greets Sir Rod Stewart appears vague, it looks to be his favourite tipple.

With the King reportedly enjoying a martini before bed, Tina Brown, the author of 'The Palace Papers: Inside the House of Windsor', said that Charles enjoys his favorite cocktail "quite wet, comprised of equal portions of gin and dry vermouth".

The King's appearance at St James's Palace in London comes just days before his busy schedule resumes.

King Charles is set to appear at this year's Trooping the Colour on June 15, with the Garter Day service taking place two days later.

He began making public appearances back in April, following medical advice that he could resume his royal duties.

The King revealed his cancer diagnosis on February 5, shortly after a hospital stay for an enlarged prostate - something that was unrelated to his diagnosis.

At the event, as he spoke about the honour of being a knight for the Foundation, Sir Rod Stewart has made a cheeky swipe at David Beckham by saying that his Knighthood honour is "coming soon".

David Beckham, who was present at today's event, has previously been accused of "begging for a knighthood" - with a previous Instagram post following the Coronation leaving fans speculating if he had an ulterior motive.

Marking the Coronation last May, David wrote: "Today our ccountry comes together to celebrate the coronation of Their Majesties King Charles III and Queen Camilla @theroyalfamily."

The caption was shared alongside an official picture of King Charles.

While delivering a speech at the prestigious event, Rod made a cheeky error which left attendees in hysterics.

Speaking of The King's Foundation, Rod accidentally referred to it as a 'Trust' - which he later apologised for.

Rod, 79, was also joined the star-studded event by his wife, Lady Penny Lancaster.