King Charles was due to meet EU Commission head Ursula von der Leyen in final part of NI Protocol negotiations
King Charles was lined up by Number 10 to meet European Commission President Ursula von der Leyen on Saturday - just as highly contentious negotiations over the Northern Ireland Protocol were poised to come to a head.
After weeks of fraught talks and growing anger among Tory backbenchers and unionists, a deal between the UK and EU over Northern Ireland's post-Brexit agreement, which could prompt a major rebellion, is expected within days.
Now Sky News can reveal that the King had been due to play a major role in the final part of the protocol negotiations, by meeting Ms von der Leyen on Saturday.
There is no suggestion the King would have taken part in the talks.
But, this could have been interpreted as giving his blessing to the negotiations at the late stage or even endorsing the deal altogether if it had concluded on Saturday morning as had been rumoured at one stage.
However, it was cancelled earlier on Friday.
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At one stage, the plan for Saturday was for Ms Von der Leyen to meet Prime Minister Rishi Sunak to endorse the deal and separately for her to meet the King.
Discussions were also had about calling the deal "the Windsor Agreement", lending an air of royal authority to it.
However, support from the King would likely be deeply controversial, risking allegations he is being dragged into politics.
His mother, the late Queen, faced criticism for a tentative step into politics when she suggested that voters in the Scottish referendum think very carefully before casting their vote.
The King meeting the negotiator at this stage could be seen as an even more overtly political act than that.
The decision to arrange the meeting would have been agreed by Downing Street and Buckingham Palace jointly.
Following Sky News revealing the plan, UK government sources said Ms von der Leyen was no longer expected to travel to the UK on Saturday despite Mr Sunak and her having "positive" talks on the deal on Friday.
They stressed it would not have been improper for the King, as head of state, to have met 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.
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Some around Mr Sunak would have been keen that the King, as perhaps the most prominent unionist in the country, was seen taking an interest in its outcome and this in turn encouraged support from unionists.
No formal invitation was ever offered. However, UK politicians and official and senior EU figures were aware of the event that had been due to take place at Windsor on Saturday afternoon.
One source close to the negotiation says that the King's involvement was definitely an attempt to sell the protocol domestically.
However, other government sources are deeply concerned that this politicises the new King even before his coronation.
Former cabinet minister and leading Brexiteer Jacob Rees-Mogg told Sky News: "If there were a plan to bring the King in before there is domestic political agreement, it would border on constitutional impropriety."
Other parties said even thinking of drawing the King into the negotiations was questionable and condemned the move.
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Labour's shadow Northern Ireland secretary Peter Kyle said: "Sunak is already showing that negotiation is better than fighting but his lack of political acumen and judgement is once again undermining his chance of success.
"It seems that there is panic in Number 10 and they are reaching for all sorts of quite absurd policies and levers to try and pull.
"I do not know how a thought of involving the King could pass somebody's mind and reach it to their mouth before they realise that this is a very very unwise policy to choose because it has constitutional implications.
"It certainly would be very highly insensitive to the politics of Northern Ireland and it certainly is nothing we should ever be involving His Majesty in."
Number 10 and Buckingham Palace declined to comment.