The Prince said his grandmother was a strong supporter of his desire to serve on the front line despite his position.

He said the monarch had been "relieved" when it was agreed he would finally be able to do the job he had trained to do.

The move was cloaked in secrecy, to avoid a repetition of the media exposure seen earlier last year when he was due to go to Iraq leading a Blues and Royals reconnaissance troop.

He said the Queen witnessed first hand his frustration at the time.

"She and everybody else did," he said. "There was a lot of frustration but as they say in the Army: 'Turn to the right and carry on'."

This time only a close circle of family and friends and as few as 15 Ministry of Defence officials were told in advance, to avoid making him or those around him more of a target. But among those kept informed from the beginning was the Queen.

"We've had a lot of talks about it since April, the last time it was supposed to happen," he said in an interview given before he set off. "She was very 'pro' me going then so I think she's relieved that I get the chance to do what I want to do."

He added: "She's a very good person to talk to about it, her knowledge of the Army is amazing for a grandmother - I suppose it's slightly her job."

Asked whether he had told his grandmother of the final decision to send him to Afghanistan he revealed: "She actually told me. She told me I'm off to Afghanistan so that was the way it was supposed to be."