Prince Philip was "determined" to save Charles and Diana's marriage with "tough but tender" letters, his biographer has claimed.

The Duke of Edinburgh and the Queen stepped in to suggest their son and daughter-in-law made a compromise when their relationship was on the rocks, according to royal biographer Gyles Brandreth.

Brandreth told the Mail Online how Prince Philip went about trying to reconcile the couple '"with home truths", inviting Diana to think about her marriage "long and hard".

The Prince and Princess of Wales
The Prince and Princess of Wales needed help from Prince Philip, Gyles said

Gyles said: "Some of his remarks were undeniably negative, but his underlying message was always positive. As for his purpose, it was characteristically practical: to guide her towards the possibility of a reconciliation with Charles.

"In one letter, he even drew up a numbered list of things – activities, interests, people – that the pair of them had in common. I doubt that the trained marriage counsellors could have done very much more.

"To be fair,’ acknowledges (former butler Paul Burrell), "Prince Philip did more to save the marriage than Prince Charles.'"

Princess Diana with Prince Philip
Princess Diana with Prince Philip on Derby Day

The Duke of Edinburgh died at the age of 99 on Friday and his funeral is set to take place on Saturday.

The funeral will be held at St George's Chapel on the grounds of Windsor Castle on Saturday and will coincide with a national three-minutes' silence.

Under government guidance, which changed from Monday, funeral ceremonies must have no more than 30 people attending, whether indoors or outdoors.

While the final guest list has not yet been announced Prince Charles is expected to attend.

On Saturday only a handful of mourners from the Royal Family will be permitted to attend the event at St George’s Chapel at Windsor Castle.

Because of laws in place to battle the spread of coronavirus anyone attending a funeral must stay at least two metres apart from anyone not in the same household.

It was previously reported that the Queen will likely sit on her own during the funeral of her husband because of strict Covid rules.