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.

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.

But, it was cancelled earlier on Friday.

At one stage, the plan for Saturday was for Ms Von der Leyen to meet Mr 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.

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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 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.

Some around Rishi 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.

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.”

Number 10 and Buckingham Palace declined to comment.

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