Harry and William: 'Dad makes us cringe'

Charles has been known to make William and Harry squirm by laughing in all the wrong places. Photo / Getty

He is, officially, His Royal Highness Prince Charles, Prince of Wales, Earl of Chester, Duke of Cornwall, Duke of Rothesay, Earl of Carrick, Baron of Renfrew, Lord of the Isles and Prince and Great Steward of Scotland.

But to his sons he is just another embarrassing dad.

Charles has been known to make William and Harry squirm by laughing in all the wrong places, as the Princes reveal in an intimate new documentary with Ant and Dec - who speak of the time they spent with the Royals in an exclusive interview with Event magazine today.

In the 90-minute film, William and Harry talk candidly of their father's habit of 'rabbiting on' and recall how he used to embarrass them by laughing at the wrong moments when he would visit them at school.

Prince William recalls one incident when the special effects malfunctioned while he was appearing in a school production - causing both Prince Charles and the Duke of Edinburgh to guffaw. He says it is a family trait to find it amusing 'when things go wrong'.

William and Harry also describe how Charles will laboriously write vast numbers of letters by hand from his study - but say his writing gets harder to read towards the end of the day as he becomes more tired. The pair often struggle to work out if they are being praised or told off. Letters Charles sends to Government Ministers are famously known as 'black spider memos' because of his handwriting.

Charles has seldom appeared more relaxed on camera than he does in the programme, When Ant And Dec Met The Prince: 40 Years Of The Prince's Trust. In it, he admits that he has yet to change one of his grandchildren's nappies, but takes comfort in the fact they will be around 'to look after me when I am doddering about'.

Ant and Dec, who are ambassadors for the Prince's Trust, spent 12 months following Charles, but in their Event interview, the Tyneside-born duo reveal they had some difficulties communicating with him. 'I think he may have had a few issues with [my accent],' Ant explains. 'I would ask him a question and sometimes he would just smile.'

Ant admits, too, that protocol occasionally became an issue. 'I found myself getting a bit over-excited and grabbing His Royal Highness and slapping him on the back... I had to keep stopping myself,' he says.