He also referred to Camilla, Duchess of Cornwall who has never gone as The Princess of Wales despite her right to use the title.
Mr MacMarthanne said: ”Consider the case of the Duchess of Windsor, or the Duchess of Cornwall, by law the former should have been an HRH and the latter the Princess of Wales, but neither were what they should have been.
“This is the nature of the elasticity of our constitution, it moves to accommodate as and when required.”
While it would be extreme for Meghan and Harry to lose their HRH styles in future, it remains constitutionally possible.
Mr MacMarthanne added: “Consequently, what has been given, can be taken away – as it was for many former HRH’s who lost the style by the 1917 Warrant, it can also be given, or whatever practices might have been can modified.”