UK Prime Minister Rishi Sunak has had his political judgement criticised, after a meeting between King Charles III and the leader of the EU was cancelled.
European Commission president Ursula von der Leyen had been expected to travel to the UK on Saturday, as rumours grew that a deal to fix the Northern Ireland Protocol was close.
The Northern Ireland Protocol is a trading arrangement that was negotiated before Britain left the EU. The UK government wants to make changes to the pact.
Ms Von der Leyen was due to meet the king at Windsor Castle, Sky News said.
There were talks about calling a potential protocol pact the "Windsor Agreement" following such a meeting with King Charles, the broadcaster said.
Government sources confirmed that Ms von der Leyen's trip was cancelled, but it is likely Downing Street and Buckingham Palace worked together on the reported arrangements.
Mr Sunak's critics said the scheduled meeting brought into question his handling of the protocol negotiations.
'On the borderline of constitutional propriety'
Conservative Eurosceptic and former business secretary Jacob Rees-Mogg told The Times newspaper the planned meeting with the king was "on the borderline of constitutional propriety".
Mr Sunak is aiming to glean the support of not just his own Conservative MPs, but also those of the Democratic Unionist Party (DUP) for any protocol reform, as he looks to restore power-sharing in Northern Ireland.
The DUP is refusing to take part in Stormont's cross-community devolved government in protest against the impact that the Brexit treaty is having on trade between Northern Ireland and the rest of the UK.
Former DUP deputy leader Lord Dodds said meeting Ms von der Leyen would have politicised the monarch, and argued the reports "reinforce the questions about Downing Street's political judgment over the protocol".
Sammy Wilson, a DUP MP, said any Windsor meeting with the EU leader would have been "a cynical use" of the king's position and seen in Unionist circles as the sovereign endorsing the deal.
He said the Prime Minister was "naive" and accused him of "dragging the king into a hugely controversial political issue".
"The only conclusion we can come to is he [Mr Sunak] knows that in these negotiations he hasn't achieved the objectives he set out for himself and his own party," Mr Wilson said on Sky News.
"Nor has he achieved the promises that he had made to ourselves and was now trying to get the king to pull the thing over the line for him."
Baroness Hoey, a Northern Irish Brexit supporter and former Labour MP, said any such meeting would have been "outrageous".
UK government sources said that, while Ms von der Leyen's trip was no longer going ahead, it would not have been improper for the King, as head of state, to meet a visiting European leader.
"It would be wrong to suggest the King would be involved in anything remotely political," a government source told the PA news agency.
Buckingham Palace would not comment.
Labour's shadow Northern Ireland secretary Peter Kyle said Downing Street should have realised utilising the King would have "constitutional implications" and been "highly insensitive to the politics of Northern Ireland".
"It certainly is nothing we should be involving His Majesty in," he told Sky.
No 10 said Mr Sunak will be spending the weekend speaking to "relevant stakeholders" as he looks to get a protocol deal over the line.
Downing Street said "intensive negotiations" with Brussels are still taking place.
The protocol, signed by former prime minister Boris Johnson in 2020, was designed to prevent a hard border with Ireland after Brexit by effectively keeping Northern Ireland in the EU's single market.
But the treaty has incensed unionists due to the trade barriers it created between Northern Ireland and the rest of the UK.