He said: “Royal non-disclosure triumphs over transparency. William’s obsession with privacy trumps any concerns as to how this may look.” He warned: “The absence of answers to basic questions could prove unsustainable in the years ahead.”

The cash from the Duchy of Cornwall is used to fund William’s Kensington Palace household and represents the personal funds for the heir to the throne. But the accounts revealed yesterday also revealed William and wife Kate embarked on a campaign of philanthropy by making undisclosed personal donations to charities linked to the war in Ukraine, as well as to earthquake appeals in Turkey and Syria. They also chipped in to make donations to the London Air Ambulance and other mental health charities.

It also emerged William was proving to be a modern Royal – by doing business over WhatsApp messages. Alastair Martin, the outgoing Secretary and Keeper of the Records for the Duchy of Cornwall revealed the Prince of Wales, 42, would communicate on the messaging platform – unlike his father, 77, who preferred phone calls.

Mr Martin said: “Sustainability is a passion that they both share but there are some differences. His Royal Highness, the 24th Duke, would telephone me. His Royal Highness, the 25th Duke, will WhatsApp me.” He added: “The day after Her Late Majesty died, I rang Prince William to say, ‘Welcome to your Duchy, Sir’.

“I obviously knew him, I’d been working with him for the previous eight years or ten years to really explain to him what his future was during the time he was heir to the throne. And he said, ‘I’m going to give you my mobile telephone number – if you want me, just get me, just message me’. And that’s how he works and he is very involved.

“There will be weekends when my WhatsApp messages will be in double figures and I will be very responsive. If something has gone well or badly, I will want to tell my boss and he’ll be straight back.”