“I am enormously proud of my sister. Her passion and drive has brought causes that are close to her heart into the spotlight, which I’m sure will make a difference in our world.”

Historian and royal house expert Sebastian Olden-Jørgensen said Margrethe II’s decision to deprive Joachim’s children of their prince and princess titles two years ago had fuelled a crisis within the family, which had not fully healed.

The Queen later issued an apology for causing her family any distress, admitting she “underestimated” how much they would “feel affected” after they expressed their hurt and surprise to Danish media.

“If you want to be a bit rude, then you can say that Joachim and Marie have chosen the minimal solution for the celebration,” Olden-Jørgensen told Ekstra Bladet.

“If you want to interpret it, it can look like a marking that ‘we know very well that our children are not princes and princesses, and they are not invited, and we still don’t think that’s right’.”

Prominent royal commentator Kim Bach said it was assumed Frederik’s brother would attend, but it was not clear why his wife would not join him.

“I assume that she has been invited, and then it may be that she doesn’t want to, or she may be prevented – she may be ill, or it may be that she has to look after the children,” Bach said.

“It is also conceivable that she still hasn’t swallowed the camel completely in relation to the children’s titles, so she can’t bear to sit with the family. However, they spent Christmas together, which speaks against that being the case. Unless Christmas was that terrible.”

Danish security agencies said on Wednesday they would be on high alert for a terror attack during the change of throne, which involves royals making trips through the streets of the capital under extensive police orders.

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Copenhagen Police inspector Peter Dahl told reporters his officers would have a “comprehensive” presence as the couple and the outgoing queen were driven through the streets to and from their residence at Amalienborg and Denmark’s parliament, where they will be officially declared king and queen.

“The terrorist threat against Denmark has been high for a long time, and this means that we have decided to make some measures in the Danish police in relation to being able to counter some of these things,” Dahl told reporters.

An official guest list has not been released, but both the British and Swedish royal families have confirmed they would not participate in the royal succession in Denmark. The royal house would not comment on whether other European royal houses would participate.

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