Queen Elizabeth II has reportedly only ever loved the Duke of Edinburgh.

Prince Philip and Queen Elizabeth

Prince Philip and Queen Elizabeth

The 91-year-old royal married Prince Philip in November 1947 and it is believed the moment the monarch "fell in love" with her spouse aged 18, she "never" looked at any other man but him.

The author of 'Elizabeth the Queen' Sally Bedell Smith told PEOPLE: "She fell in love at age 18 and she never looked at anyone else."

And while the popular Netflix series 'The Crown' depicts the reservations Elizabeth's father King George VI had about his daughter's partner, her relationship is rumoured to be one of the Queen's "greatest achievements".

The programme's executive producer Suzanne Mackie said: "One of Elizabeth's greatest achievements is being allowed to marry the love of her life. Like any marriage, it would undertake endless recalibration and navigation and re-negotiation."

And the 95-year-old royal - who announced he was retiring from royal duties on Thursday (04.05.17) - is believed to have been "very much in love" with Elizabeth as well when they first set their sights on one another and exchanged letters to eachother when she was only 13.

The late Sir Edward Ford, who was the assistant private secretary for 15 years, previously said: "Some were very concerned [about Philip]. The line was slightly tenuous at that point. So, it was only natural that the older generation -- friends of the King like Lord Salisbury -- were concerned that who the Queen was with was totally and utterly suitable. So they were sniffing around to see what he was like.

"[But] he was a perfectly natural young sailor and very much in love with the girl of the house. But he would not in any way fawn on the elders and say, 'What a suitable husband I am going to be.' He was very much his own man."