Their close bond has sparked many rumours over they years - and now the relationship between Prince Philip and confidant Penny Knatchbull is explored in the controversial new episodes of The Crown.
The pair, who are said to have enjoyed decades of friendship, are portrayed in the Netflix hit as touching hands while Philip, played by Jonathan Pryce, moans to Penny, played by Natasha McElhone, about his marriage to the Queen.
Although the show does not depict the pair having an affair - it strongly implies their bond might have been more than friendship. However, it is something that has always been denied - including by Philip himself - and The Crown has been heavily criticised for embellishing and dramatising storylines for entertainment purposes.
But what exactly was the bond like between the pair, how did they become friends and what did the Queen think of their closeness? Here we take a look...
Philip and Penny, who is 32 years younger than the late Duke of Edinburgh, are said to have first met in 1975 when she became engaged to Norton Knatchbull.
He is a grandson of Lord Mountbatten, who was Philip's uncle and was also portrayed in previous series of The Crown by Greg Wise and Charles Dance.
The wedding between Penny, who is also the Countess Mountbatten of Burma, and her husband was delayed following the murder of Lord Mountbatten in 1979 in Ireland at the hands of the IRA.
But when it did take place in 1979, King Charles, who was Prince of Wales at the time, was best man for the groom.
Bonded by grief
Penny and Norton had three children together Nicholas, Alexandra and Leonora, who was born in 1986. However, in 1991, Leonora died after a battle with kidney cancer at the age of just five.
After the little girl's death, the Queen and Philip supported the family, with the then-monarch's husband understanding loss better than most due to a childhood filled with tragedy, including the deaths of his sister and two nephews in a plane crash.
In a friendly gesture, Philip then taught Penny how to carriage ride in 1994, knowing that she and he shared a love of the outdoors - and she later became his driving partner, joining him at competitions.
Penny's marriage hit rock bottom after her daughter's death when her husband went to live in the Bahamas with a fashion designer - leaving her to maintain her husband’s vast estate and 18th-century Broadlands home.
Royal expert Ingrid Seward told The Sun : "Running royal properties was something Philip knew full well and having bonded with her over horses and carriage driving, their friendship blossomed further.
"While driving around sprawling country estates, Philip liked to discuss religion and other deep, worldly matters.
"I'm sure Penny, who was educated in Switzerland and the London School of Economics, would have indulged his curious mind. "
But due to their close nature, their bond was marred by rumours of a romantic relationship
However in 1992, Philip once told a journalist: "'Have you ever stopped to think that, for the last 40 years, I have never moved anywhere without a policeman accompanying me?
"So how the hell could I get away with anything like that?"
When Philip moved to Sandringham after his retirement in 2017, Penny was a regular guest at his Wood Farm cottage.
Staff nicknamed her "and also" as Philip also insisted that she was added onto the guestlist for all events.
They enjoyed each other's company and would regularly attend the Royal Windsor Horse Show together.
But it wasn't just Philip who was friendly with Penny - she also enjoyed a close relationship with the Queen, with the pair sharing a mutual love of horses and also being photographed together in high spirits at the Royal Windsor Horse Show.
When Philip died in April last year at the age of 99, Penny was one of the just 30 mourners who were invited to his funeral, which was scaled back due to Covid restrictions.
A source told Grazia : "Penny and the Duke of Edinburgh shared decades of affection. Her presence at the funeral among only thirty attendees is exactly what the late Duke would have wanted, and the entire family will be pleased to have her among them on the sombre occasion."
Meanwhile in September, following the death of the Queen, Penny also attended her state funeral at Westminster Abbey alongside her daughter Lady Alexandra.
Since news emerged that the 'affair' storyline between Philip and Penny would be explored in season five of The Crown, there has been fury.
The Queen's former press secretary Dickie Arbiter told The Sun : "Coming just weeks after the nation laid Her Majesty to rest next to Prince Philip, this is very distasteful and, quite frankly, cruel rubbish.
"The truth is that Penny was a long-time friend of the whole family. Netflix are not interested in people’s feelings."
While Ingrid added: "I’m sure this new storyline for The Crown has caused untold stress and heartache for Penny, her children and her wider family.
"And in so many ways it seems unjustified – she is the unfortunate victim in their fictional tale because she was young, attractive, and photographed with Philip."
A spokeswoman for The Crown said: "The Crown has always been presented as a drama based on historical events.
"Series five is a fictional dramatisation, imagining what could have happened behind closed doors during a significant decade for the royal family – one that has already been scrutinised and well-documented by journalists, biographers and historians."