The King arrived at Blenheim Palace as Sir Keir Starmer was hosting a meeting of European leaders.
The monarch is due to receive around 50 leaders, including French President Emmanuel Macron and Italian leader Giorgia Meloni, who have gathered for the European Political Community at the historic site of Woodstock, Oxfordshire.
Sir Keir received the King on his arrival at the Great Court at Blenheim Palace, before the King proceeded to Long Library for the reception.
The library has a collection of over 10,000 books and is also home to the magnificent Willis Organ, commissioned in 1888 by the 8th Duke of Marlborough.