Von der Leyen is to hold talks with Prime Minister Rishi Sunak, after which they are expected to announce new trading terms to resolve issues over the British province. The commission confirmed she would also meet Charles.

Some politicians raised concerns that the monarch, who constitutionally is not supposed to be involved in political matters, was being pulled into the Brexit debate by the government, which could be creating an impression he was endorsing a deal by agreeing to meet the European Commission president.

"The king is pleased to meet any world leader if they are visiting Britain and it is the government’s advice that he should do so," a palace spokesperson said.

A palace source said the meeting between Charles and von der Leyen followed government advice that her visit would provide an opportunity to discuss a broad range of issues including Ukraine and climate change.

Sunak's spokesman said it was not unusual for the government to provide the monarch with advice.

"It is a matter for the palace to decide, and ultimately for the king to decide, if he wants to meet individuals," the spokesman said. "His majesty has met with a number of foreign leaders recently, this is no different."

A Commission spokesperson said the Sunak meeting would discuss the Northern Ireland protocol, while discussions with the king were "not part of this process".

The success of any deal to sort out tensions caused by the 2020 post-Brexit arrangements governing the province could hinge on whether it convinces the Democratic Unionist Party (DUP) to end its boycott of Northern Ireland's power-sharing arrangements.

However, the decision to arrange a meeting with Charles was not popular with some in the DUP, with former leader Arlene Foster calling it "crass" and saying it would not go down well in Northern Ireland.

"I cannot quite believe that No10 would ask HM the King to become involved in the finalising of a deal as controversial as this one," she said on Twitter.

"We must remember this is not the King’s decision but the Government who it appears are tone deaf."

Opposition Labour lawmaker Chris Bryant said it was a "terrible mistake".

"We should never bring the monarchy into political disputes," he said on Twitter.