Kate and William urged to follow Harry and Meghan by turning their backs on royal life

Smith discussed the Prince and Princess of Wales potentially following the example set by the Duke and Duchess of Sussex in 2020, when they stepped back as senior working royals.

When asked if he would want William, 41, and Kate, 42, to "follow a similar path to Harry and Meghan", Smith said: "Yes, but I think they all would.

"At the moment we pay through the Duchy of Cornwall, which is not private property despite their claim, we pay William a personal income of about £25million a year, which is six times the combined incomes of all the elected heads of state of Europe at least.

"For that, he does very little. If you look at his calendar, his schedule of engagements, which are typically an hour long, if that, it really doesn't add up too much."

Cameron interjected by saying: "Sorry to interrupt you Graham, but obviously he's only seen in public for an hour or two a day, I accept your point on that. But there's a lot of work that goes on behind the scenes."

However, Smith claimed: "Well, I dispute that because there's no evidence of this work. Other people do lots of work behind the scenes.

"There's no evidence that he does. And a lot of what he puts on the engagement schedule is also stuff that he does behind the scenes, including Zoom calls and telephone calls.

"If he's putting meetings of the Duchy of Cornwall, which he claims is private, on his list of engagements, they're padding it a little bit.

"But the point is, we don't get an awful lot in return for £25million or £24million, which is an obscene amount of money to throw at one person.

"So we would all be better off, including William and Kate, if they went off and did their own thing, whether they choose to live in the States or stay here is entirely up to them.

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"But I just don't see any point in us throwing an estimated £345million plus at the royals, which is the more accurate figure than the sovereign grant, when they do so little, when we could actually just pay a normal salary e.g. the salary of the Prime Minister to one person to do a very serious job of being head of state."

Harry, 39, and Meghan, 42, stepped back as senior royals in 2020 following an 18-month stint as a working royal couple.

The Duke and Duchess of Sussex now reside in the US with their children, Prince Archie, five, and Princess Lilibet, who will turn three next week.

Meanwhile, William and Kate live in Windsor with their three children, Prince George, 10, Princess Charlotte, nine, and Prince Louis, six.