Royals Prince Edward and Sophie Stand in for King Charles During a Royal First at Buckingham Palace The Duke and Duchess of Edinburgh watched French soldiers take part in the Changing of the Guard for the first time By Simon Perry Simon Perry Simon Perry is a writer and correspondent at PEOPLE. He has more than 25 years’ experience at PEOPLE covering the royals, human interest and celebrity. People Editorial Guidelines Published on April 8, 2024 08:42AM EDT Sophie, Duchess of Edinburgh and Prince Edward at the Changing of the Guard in London. Photo: VICTORIA JONES/POOL/AFP via Getty Prince Edward and Sophie, the Duchess of Edinburgh took part in a historic Buckingham Palace ceremony on Monday as King Charles continues his treatment for cancer. Tourists watching the traditional Changing of the Guard through the railings witnessed the unique treat of watching French troops join their British counterparts in an unprecedented move for the daily ceremony. King Charles asked his brother Edward, 60, and sister-in-law Sophie, 59, to stand in for him at the special parade in honor of the 120th anniversary of a key diplomatic agreement between the U.K. and France known as the "entente cordiale." Sophie, Duchess of Edinburgh and Prince Edward at Buckingham Palace on Monday April 8. VICTORIA JONES/POOL/AFP via Getty Prince Edward Makes a Quip About His Age Before Turning 60 as Sophie Praises 'Darling' Husband in Sweet Speech Since his cancer diagnosis, which was announced on Feb. 5, King Charles continued to work behind the scenes but has not been able to carry out most of his public duties. On previous occasions, his wife Queen Camilla, 76, has stepped in for him, but he asked his youngest sibling and Sophie to do so on Monday. Standing on the forecourt, alongside dignitaries from Britain and France, the royal couple helped mark the 120 years since the signing of the "entente cordiale," which laid the foundation for the military and diplomatic cooperation between the two countries during the First and Second World Wars — and which continues to the modern day. Prince Edward inspects France's Garde Republicaine as they took part in the Changing of the Guard at Buckingham Palace on Monday April 8, 2024. VICTORIA JONES/POOL/AFP via Getty Shortly after arriving at the palace forecourt, Prince Edward, Sophie and the French ambassador to the U.K., Her Excellency Hélène Duchêne, inspected the troops. In front of them, 32 members of the Gendarmerie’s Garde Républicaine joined 40 Guardsmen from the Scots Guards. It was the first time that soldiers from a non-Commonwealth country had joined the ceremony. In a parallel move, 16 soldiers from Number 7 Company Coldstream Guards, accompanied by two military musicians, joined troops from 1st Régiment de la Garde Républicaine to provide the Presidential Guard outside the Elysée Palace, in Paris. Like London, it was the first time a foreign state helped guard the French Presidential Residence. The troops were inspected there by French President Emmanuel Macron. Sophie, Duchess of Edinburgh and Prince Edward at Buckingham Palace in London on Monday April 8, 2024. VICTORIA JONES/POOL/AFP via Getty Can't get enough of PEOPLE's Royals coverage? Sign up for our free Royals newsletter to get the latest updates on Kate Middleton, Meghan Markle and more! On Sunday, King Charles was seen heading to church at Sandringham, Norfolk. He has been staying positive during his treatment and aides have indicated that he might be able to carry out more public events in the coming weeks and months, depending on advice from doctors.