Joining the Royal Family as an ‘outsider’ himself, Prince Philip could understand the challenges Princess Diana was facing.

Diana and Philip – or ‘Pa’ as he was known to his daughter-in-law – exchanged a series of letters in 1992 through which the Duke of Edinburgh attempted to counsel the Princess, signing off each letter with a touching message.

The letters became a focus of Diana’s inquest in 2008 when complementary therapist Simone Simmons claimed that two of the letters were ‘cruel’ and ‘upset’ the young royal.

This, coupled with anger towards the Royal Family for their delayed response to Diana’s death, fueled the public perception that Philip was unsympathetic to his daughter-in-law.

Broadcaster and writer Gyles Brandreth is one of the best-positioned people to discuss the late Duke of Edinburgh, having known him for more than 40 years.

He met the Duke when he became involved in the work of the National Playing Fields Association, of which Philip had been president since 1948.

He once described Philip as a ‘funny man who liked to laugh and make others laugh’.

‘I very much enjoyed his company,’ he continued.

‘Having observed him many times at close quarters, I also noticed that the more unassuming people were, the friendlier he’d be.’

In his 2021 book, Philip: The Final Portrait, Brandreth wrote that not long after Diana’s death, he said to the Duke of Edinburgh: ‘The public view of you, for what it’s worth, is of a grouchy old man, unsympathetic to his daughter-in-law.

‘But I happen to know – not from you, but I know it – that when things were difficult, you wrote to Diana – kind letters, concerned, fatherly, loving letters from Pa, explaining how you knew, first hand, the difficulties involved in marrying into the Royal Family.’

‘The impression the public has got is unfair,’ Brandreth reportedly told Philip.

Philip is said to have shrugged and responded: ‘I’ve just got to live with it. It happens to a lot of people.’

In 2002, a Palace statement read that the original letters sent by Philip to the Princess had been lost, but he kept copies of his letters to her and he also had the original replies from the Princess.

Former royal butler Paul Burrell is said to have leaked some of the letters in 2003, and at the time, The Sunday Times reported that Philip was attempting to block them being published.

Brandreth wrote: ‘Philip’s letters to Diana were typical of his correspondence overall.

‘They were sympathetic, but unsentimental, direct, but to a purpose.

‘In his letters Philip confronted his daughter-in-law with home truths: he invited her to face the facts.

‘Essentially he wanted to make Diana think about her marriage, long and hard. And he did.’

All of Philip’s letters to Diana he signed ‘With fondest love, Pa’, and he always referred to himself and the Queen as ‘Pa and Ma’.

For her part, Diana’s flow of letters to Prince Philip were filled with the pain and frustrations of a young married mother whose husband’s ardour was focused on another woman.

She showed his letters to several of her friends who helped her construct replies that were as unblinking as his had been.

Brandreth wrote: ‘When Diana received the letters she was at her most vulnerable and volatile.

‘As soon as one arrived, she opened it, scanned it, usually burst immediately into tears and then shared it, as soon as possible, with her closest friends.

‘Rosa Monckton, then managing director of Tiffany’s in London at the time, and Lucia Flecha da Lima, wife of the Brazilian Ambassador, were probably Diana’s two closest girlfriends at the time of her death.’

‘They were good letters. He is a good man,’ Lucia apparently told Brandreth.

‘Actually, he was pretty wonderful.

‘All he was trying to do was help. And Diana knew that,’ Rose said.

This remarkable exchange of letters ended when the princess and Charles separated in December 1992.

Diana continued to see both Ma and Pa at formal royal occasions, Philip always taking an interest in what she was doing.

In the aftermath of the inquest into Diana’s death, the Daily Mail’s Richard Kay wrote: ‘Diana had come to realise just how much she admired Prince Philip.

‘And he, a man who had lived more than a bit, and understood a lot, had come to recognise why he felt such a well of sympathy for her.’

After Diana’s death, Philip was said to be a guiding presence for the young Princes William and Harry, offering them counsel and support.

On his death, William paid tribute to his grandfather, saying Philip’s ‘enduring presence’ guided him ‘both through good times and the hardest days’.

He shared that his own children would cherish memories of their ‘great-grandpa’.