The Prince of Wales has appointed a former diplomat as his private secretary, as he returns to royal duties after his father’s cancer diagnosis.

Ian Patrick, who has also worked for the former Liberal Democrat leader Paddy Ashdown, would take up the role, Kensington Palace said.

The prince resumed regular duties this week having taken time off to support his family after the Princess of Wales had planned abdominal surgery.

Prince William hosted an investiture ceremony at Windsor Castle on Wednesday before attending the London air ambulance charity gala dinner later that evening, thanking the public for their messages of support for the king.

Addressing guests at the dinner, he said: “I’d like to take this opportunity to say thank you, also, for the kind messages of support for Catherine and for my father, especially in recent days. It means a great deal to us all.”

It was announced on Monday that Charles would have regular treatment for an unspecified cancer, found during a recent hospital stay for a procedure on an enlarged prostate.

William was expected to take over some engagements as the king’s diary was hastily rearranged and as Catherine recovers at Adelaide Cottage in Windsor. He had said he would return to work only once the princess’s care and recovery had settled. Catherine, 42, is not expected to resume official duties until after Easter.

Patrick worked under Lord Ashdown when he was in Bosnia from 2002 to 2006 before joining the Foreign, Commonwealth and Development Office. He is a trustee of the charity Crohn’s & Colitis UK.