Following years of frosty relations and a tell-all memoir, the King's relationship with Prince Harry could finally be thawing – but royal experts say there is one key player who could make or break a more public reunion.

As the monarch's wife – and the Duke's stepmother – Queen Camilla could apparently be the influence which sways how Charles and his second son progress in the rehabilitation.

Speaking to the Daily Mail, royal author Richard Fitzwilliam explained how Her Majesty could be the deciding factor on whether the King will attend Harry's passion project; the Invictus Games in Birmingham next year.

'The vast coverage the royal family's rift with the Sussexes has been getting recently has tended to focus on King Charles and his attitude to reconciliation which differs from his heir [Prince William] as well as the chaos over the Sussexes schedules and their visit to Althorp,' he shared.

'Yet when he met the Sussexes for tea Queen Camilla was in attendance. She has no reason to be fond of Harry as his attacks on her in Spare and the interviews publicising it must have been wounding.

'Did the world really need to know that he and William begged Charles not to marry her? He claimed he was "sacrificed" on her personal PR altar.'

However, Richard noted: 'It is also true Harry said he had "a huge amount of compassion" for her, bearing in mind what she had gone through and did not regard her as an "evil stepmother".'

'However his comments, verbal and written were reportedly the reason for the Sussexes eviction from Frogmore House by an angry King. Camilla, as she always has in difficult situations, has stayed mute.

'However, by enduring the slings and arrows of popular opinion so stoically after Diana's death, she achieved dignity.

'She was always liked by those that knew her and her remarkable qualities, her stoicism, humour and loyalty as well as her commitment to several causes, have gradually become more widely appreciated.

'Harry will be well aware of the trust his father places in her.'

The Duke's reported ambitions to have the King to open the Invictus Games in Birmingham next July could, hence, rely on how much Harry and Camilla's dynamic could thaw too.

'It is possible that, in a show of solidarity, with her 80th birthday the week after a cause for celebration, Queen Camilla will be with him,' Richard continued.

'This would not mean that she trusted Harry, but it would mean that she believed that the monarchy and the King's reputation would be best served by appearing. No one can deny that Invictus is an important military charity, it celebrated 10 years with a Service at St Paul's.

'If the King did not attend he could be criticised for putting his personal views and those of William, in front of his role as Commander in Chief of the Armed Forces. 

'Queen Camilla's influence with the King is likely to weigh heavily with his decision. Her viewpoint would never be expressed publicly. She never spoke publicly during the years she was pilloried by Diana's supporters.

'This means that, after their last meeting, the Sussexes had better keep very quiet, avoiding the press, as undoubtedly was arranged.

'If she judges it to be his duty or in the monarchy's interest Camilla might well advise the King to attend.

'However Harry and Meghan had better avoid any contact with the press about sensitive issues in the meanwhile.

'Camilla knows the importance of family, and has previously been instrumental in encouraging harmonious relationships between Charles and William and with the Waleses children, though this was not problematic.

'She knows the King suffers from cancer and is of advanced years so his rule may not be a long one . He will have a legacy and she will, I think, see reconciliation as important as future generations may wonder why those involved in the rift did not try harder.'