The second in line to the British throne presented the actress with the British Academy of Film and Television (BAFTA) Fellowship Award and made his quip while introducing her, referring to Helen's famous portrayal of Queen Elizabeth - his grandmother - in 'The Queen'.
Speaking at London's Royal Opera House, where the awards took place, he said: ''The recipient of this year's fellowship is an extremely talented actress who I should probably call granny.''
William's co-host, actor Jeremy Irons, added: ''Helen Mirren has always been an actress impossible to ignore.
''Everything she did was always worthwhile, it was always serious, and because of her it was always intrinsically sexy.
''She has 11 BAFTA nominations and has won four, she is one hell of a dame and of course she should be a BAFTA fellow.''
Helen, 68, then collected her award, which she dedicated to one of her schoolteachers, whom she said was a huge influence on her but had sadly passed away recently.
She added: ''My little life is rounded with this honour, thank you very much indeed.''
Helen later said it was the first time she had met William.
She said: ''I wanted to take out a hankie and rub it on his face. It was an honour and he's really lovely. He's really very sweet.''
The BBC drama starring Aidan Turner returns to BBC One on September 4th.