Prince William at centre of dramatic royal U-turn as Queen protected him from 'great risk' Prince Harry faced

General Sir Mike Jackson tells an ITVX documentary called The Real Crown that the risk of war was deemed too great for the heir to the thrown.

However, Prince Harry was allowed to fight in Afghanistan as the Duke of Sussex was not in the direct line of succession.

General Sir Mike Jackson said: "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.

"She [The Queen] 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 William as heir to the heir, the risk is too great.

"But for his younger brother, the risk was acceptable."

Sir John Scarlett, then head of MI6, gave an insight into the Queen's role in making such pivotal decisions.

He said: "Of course she has complete clearance to everything.

"She has complete access to an exceptional amount of info and insight for longer than anyone else.

"She’s very, very discreet, completely reliable and completely on top of the detail.

"I remember thinking at the time, ‘Wow, Her Majesty knows more about this than we do’."

Mark Cann, director of the British Forces Foundation, tells the series that Prince William wanted to go to war.

He said: "William was very keen to go. Unequivocally.

"But it was complex, and some very great minds and experienced people took a view on it.

"I think it was really tricky. Anybody who’s in the military who hasn’t actually been on operation feels a sense of disappointment.

"And I think especially that was the one (war) at the time, you’ve got everyone around you at the time who’s been involved in it. So there is a sense of disappointment."