The late Queen authorised the Prince of Wales and Duke of Sussex to fight in Afghanistan saying “my grandsons have taken my shilling, therefore they must do their duty”, claims a new documentary.

General Sir Mike Jackson reveals Queen Elizabeth’s comments in an ITV series chronicling the royal family, but says it was decided William, as a future king, would not serve on the front line.

The Real Crown: Inside The House Of Windsor is a five-part series which can been viewed in full from April 20 and which features interviews with contemporary figures.

General Sir Mike Jackson, who served as Chief of the General Staff (CGS) from 2003-06, says in the final documentary episode: “The chiefs have an audience with the Queen once or twice a year. You need to have done your homework.

“She’s very wise. I used to tell my staff, see if you can get the midday slot because after half an hour or 40 minutes the Queen would ring a small bell and ‘Time for a sherry I think, CGS’. ‘Very good idea, ma’am’.

“What goes on in those audiences and who says what to whom, remains for the two people involved.

“And I will break the rule … about not divulging what goes on on this one occasion when she was very clear, she said, ‘My grandsons have taken my shilling, therefore they must do their duty’.

“And that was that. But it was decided that for William as heir to the heir, the risk is too great. But for his younger brother, the risk was acceptable.”

Harry was an army officer and William an officer cadet when Sir Mike was head of the army, with Harry going on to serve two tours in Afghanistan where UK forces were part of a multi-national operation fighting the Taliban insurgency.

Harry’s first tour of Afghanistan, during 2007-08, saw him work as a forward air controller co-ordinating air strikes on Taliban positions.

His tour of duty was abruptly ended when foreign websites broke a media blackout on reporting details of his service.

With a burning desire to return to the front line the royal retrained as a helicopter pilot and was deployed back to the front, serving as an Apache helicopter co-pilot gunner during 2012-13.

William served as an RAF search and rescue helicopter pilot but was not deployed into an active combat zone, unlike his uncle the Duke of York.

At age 22, Andrew saw active service in the Royal Navy as a Sea King helicopter pilot in the Falklands War in 1982. His service included flying his aircraft as a decoy target, trying to divert deadly Exocet missiles away from British ships.