William steps in for Charles as he attends graduation ceremony at RAF Cranwell

William is believed to have invited his mother’s sister to the event at RAF Cranwell, where relatives proudly watched as more 50 cadets graduated, officially becoming officers at the stroke of midnight.

The future King acknowledged the royal salute of more than 50 new officers on the parade ground and told them the country would be relying on them to deal with “future threats” during the present “time of change and uncertainty”.

It is understood Lady Sarah, who lives locally, heard her nephew would be attending the ceremony in Lincolnshire and wanted to see him and lend support.

Diana, Princess of Wales had three siblings – Earl Spencer, Lady Sarah and Lady Jane Fellowes – who gathered last month for the funeral of Lady Jane’s late husband Lord Fellowes.

William and his estranged brother the Duke of Sussex attended the funeral but did not talk to each other, according to reports.

After the graduation ceremony the prince had a private lunch and when he later met RAF graduates and their families he was followed into the room by his aunt who spent her time chatting to William’s private secretary.

It is unusual for members of the Spencer family to attend royal engagements, the last significant event was when the princess’ siblings were guests for the unveiling of Diana’s statue in the garden of Kensington Palace by her sons.

William knows RAF Cranwell well, as he trained at the air force base, flying solo for the first time and later receiving his wings from his father during a ceremony there in 2008.

He remained in the RAF for much of his active military career, going on to fly as a helicopter pilot in the RAF Search and Rescue Force before becoming an air ambulance helicopter pilot.

In a speech on the parade ground William told the graduates: “We live in a time of change and uncertainty, and you, as the future of the Royal Air Force, are the ones who will ensure that we are able to adapt and face future threats.”

The prince, who wore his RAF uniform and was still sporting his summer beard, said: “Whilst I didn’t graduate on this exact Parade Ground, I did graduate from flying training here so I know something about the celebrations that will come later, so I promise, I’ll only take a few minutes of your time.

“I do want to take this moment though, to highlight not only the importance of your achievements to date, but also that of the roles you will play in supporting the futures of your countries’ Air Forces.

“I say ‘your countries’, as I know that in addition to our newest Royal Air Force officers on parade today, there are also officers from Jordan, Kenya, Pakistan and Uganda.

“Multi-national relationships are key to international defence, security, and peace.”