Runners participating in a charity 5K in England were stunned when they realised King Charles himself was cheering them on from the sidelines.

The monarch made a surprise appearance at the park run, which was organised by a cancer support group, standing trackside to wave on participants as they completed their laps.

An organiser stood next to His Majesty holding a sign that read: “Yes, that really was the king on your first lap,” confirming what many runners thought was too good to be true.

Know the news with the 7NEWS app: Download today

The King watched those on a charity run. (AP PHOTO) Credit: AAP

The King’s appearance at the charity event comes as he continues to show support for cancer-related causes, with the park run specifically organised to raise funds and awareness for cancer support.

His Majesty was described as “resplendent” as he carried out what some might call one of his signature duties - waving to well-wishers and supporters.

The surprise royal appearance provided an unexpected boost for runners tackling the 5K course, with many doing a double take as they passed the King during their first lap.