King Charles' meeting with Ursula von der Leyen at Windsor Castle has attracted major criticism from politicians and commentators.
Former royal correspondent for the BBC — Peter Hunt — said that "heads will roll" over the "serious error in judgement" and one constitutional expert said, "all of this is unwise".
Yahoo UK explains why the decision is sparking such controversy.
'Terrible mistake' - what has the reaction been so far?
The decision has sparked widespread criticism because the negotiations surrounding Northern Ireland's post-Brexit trade position are firmly a political issue - something Charles is constitutionally bound as sovereign to stay out of.
Commentator Peter Hunt said the meeting with von der Leyen was a “serious error of judgment” adding that the “King has abandoned his unifying role and entered the political fray”.
Hunt also said that “history won’t be kind” to the move, and writing in The Spectator, said that it will likely result in the resignation of one of the King’s advisors, saying “someone’s head will roll”.
He added: "With one handshake and the click of an attendant camera, he abandoned his unifying role and entered a political fray where the Brexit dust has yet to settle."
Constitutional expert Craig Prescott from Bangor University told The Times that while on a superficial level, there is nothing wrong with Charles meeting von der Leyen but the "Northern Ireland Protocol is a matter of huge controversy."
Prescott added: "All of this is unwise. A cardinal rule is the monarch does not get involved in politics, and really you should put an electric fence around that rule."
Conservative MP Jacob Rees-Mogg has said to GB News: "The sovereign should only be involved when things are completed and accepted" and that the monarch "doesn't express his view on acts of parliament" while they are still being processed.
Chris Bryant, a Labour MP, said it was a "terrible mistake" for the King to meet with von der Leyen.
"This is a terrible mistake by the government. We should never bring the monarchy into political disputes."
Former first minister of Northern Ireland - Arlene Foster - called the decision "crass" and that it would "go down badly in Northern Ireland". She also couldn't "quite believe No 10 would ask HM the King to become involved in the finalising of a deal as controversial as this one".
Democratic Unionist Party (DUP) MP Sammy Wilson said Downing Street was "dragging the King into a hugely controversial political issue".
However, Foreign Secretary, James Cleverly, has defended the King’s meeting with von der Leyen saying it is “not unusual” for “senior international representatives” to meet the monarch while they are in the UK.
Why is it controversial for Charles to meet Ursula von der Leyen?
Many are interpreting this move as the King giving his seal of approval to the new agreement, meaning it can be seen as the politicisation of the monarch.
As a 'national symbol' for the entire UK, Charles cannot be seen as favouring the interests of individual politicians, or any particular political position: he represents the four nations equally and in their entirety, which is why it is so important he remains neutral.
If he can be viewed as 'taking a stand' in any respect then it undermines the power of the sovereign's symbolic reach as head of state and, in the long term, the stability of the monarchy.
Watch: Rishi Sunak strikes post-Brexit Northern Ireland Protocol deal with EU
Whose decision was it for the King to meet Ursula von der Leyen?
A spokesperson for Buckingham Palace has said that the King met von der Leyen on the advice of the government.
"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."
However, Downing Street said the opposite, that it was "fundamentally" the King's decision.
Sunak's spokesperson also said that the prime minister "firmly believes it's for the King to make those kind of decisions".
The Foreign Secretary also said that the “final decision” rested with the palace about whether King Charles was able to meet von der Leyen when speaking to Sky News, and when questioned on Good Morning Britain about whose idea the controversial meeting was, he said the government had “facilitated” it.
Both parties therefore, are keen to place the blame for this controversial meeting at the other's door.
This comes after reports at the weekend that Charles was due to meet von der Leyen as part of the negotiations, although it has not been suggested he would have taken part in the talks himself.
More reporting on the Northern Ireland Protocol