The three were also joined by Camilla, Duchess of Cornwall at the service held at Brentwood Cathedral in Essex. William's wife, Kate Middleton, was not in attendance.
Charles had known van Cutsem, who passed away on Sept. 2 at the age of 72, for more than 50 years, having met during their days at Cambridge University.
According to the Daily Mail, William and Harry apparently spent quite a bit of time with the van Cutsem family when the marriage between Charles and Princess Diana began to fall apart. As a result, the brothers became close with the van Cutsems' four sons. It is believed that one of them will soon be asked to be godfather to Prince George.