The duke was waiting for a review by the Risk Management Board (RMB), part of the process by which the Executive Committee for the Protection of Royalty and Public Figures (Ravec) rules on his security requirements, but found out on Friday that this has yet to take place. A spokesman for the duke said: “The duke continues to explore every available option to enable the visit to proceed safely and to give his children the opportunity to enjoy the UK.”

But the Daily Mail’s Diary Editor Richard Eden was critical of the Sussexes and said their move had an “unedifying” motive.

He said on the Palace Confidential podcast: “This trip is a big deal. It would be the first time they have been here together since the Queen’s Platinum Jubilee celebrations in 2022.

“It would only be the second time that Lilibet would have met her grandfather, the King.

“It does seem that they are using the children to put pressure on the King, and for me, that’s really unedifying, I don’t like it at all.”

The children are not expected to attend any public events, but Meghan is scheduled to join her husband on visits to the Royal Hospital Chelsea in London and an Invictus event at the National Exhibition Centre in Birmingham.

There is hope the King will be able to reunite with his youngest grandchildren for the first time in four years.

Charles has offered the Sussexes accommodation at a royal residence, but it is understood the Royal Household has yet to receive confirmation of acceptance.

It is believed the family will not receive taxpayer-funded protection while in the UK, except when within royal residences.

The spokesman for the duke added: “Prince Harry’s programme in the United Kingdom includes both public and private engagements across the country.

“Safe accommodation is only one element of an effective protective security plan because risk follows the person, not the place.

“The issue has never been accommodation. The issue is whether appropriate and proportionate protective security is being provided throughout the entirety of the visit.

“The independent Risk Management Board that Ravec itself decided was necessary last November has still not taken place.

“It is therefore difficult to understand how the proportionality of the current arrangements can credibly be maintained without that independent assessment.”