His Majesty King Charles III has just appointed his younger brother and sister-in-law, The Duke and Duchess of Edinburgh, to carry a high-level diplomatic mission abroad.

His Majesty King Charles III has just appointed his younger brother and sister-in-law, The Duke and Duchess of Edinburgh, to carry a high-level diplomatic mission abroad.

On June 1 to 3, 2026, Their Royal Highnesses Prince Edward and Sophie, the Duke and Duchess of Edinburgh, will travel to Portugal to mark the 640th anniversary of the Treaty of Windsor, considered as the oldest diplomatic alliance in the world, and celebrates the historical ties and modern relationship between the UK and Portugal.

This is a major diplomatic engagement abroad, and appointing his brother and his sister-in-law, instead of The Prince and Princess of Wales, to carry out the task, is an extraordinary decision from King Charles III.

So what is the Treaty of Windsor?

The Treaty of Windsor is a treaty of peace, friendship and confederation between King John I of Portugal (reigned: 1385-1433) and King Richard II of England (reigned: 1377-1399).

The Treaty of Windsor is a diplomatic alliance signed between the Kingdom of Portugal and the Kingdom of England on February 24, 1387 in Windsor, Berkshire, and sealed by the marriage of King John I of Portugal (House of Aviz) and Philippa of Lancaster, daughter of Prince John of Gaunt, 1st Duke of Lancaster (the fourth son of King Edward III of England).

Philippa was the older sister of King Henry IV of England, the founder of the House of Lancaster of the English throne, and first cousin of King Richard II, the last Plantagenet King.

In 1943, while speaking in the House of Commons, British Prime Minister, Winston Churchill described the Anglo-Portuguese Alliance as “an alliance without parallel in world history.”

The historian Matthew Winslett says, “This treaty has been the cornerstone of both nations’ relations with each other ever since.” Still in effect today, it is the longest-lasting diplomatic treaty in recorded history.

The Duke and Duchess of Edinburgh will visit Lisbon and Porto, and apart from the 640th anniversary celebration of the diplomatic ties between Portugal and the United Kingdom, Their Royal Highnesses will also undertake “engagements in support of issues important to them which includes youth opportunity, Women, Peace and Security.”

They will also visit historic sites that represent UK–Portugal heritage, alongside meetings with students, community organisations, and highlighting innovative partnerships.