When Rishi Sunak sought unionist backing for the new post-Brexit Northern Ireland settlement, the King's meeting with EU leader Ursula von der Leyen infuriated them.
Tory Brexiteer Jacob Rees-Mogg said Charles should not be drawn into the "current political dispute."
Baroness Arlene Foster said the meeting in Windsor Castle was "crass and will go down very badly".
Buckingham Palace said Charles was following "government advice," but Downing Street maintained it was "essentially" the King's choice.
Charles met Ms. von der Leyen at Windsor Castle, where he married the Queen Consort in 2005.
Climate change, the Russian invasion of Ukraine, and maybe Northern Ireland were discussed.
There have been rumours that the EU leader contacted Buckingham Palace directly to set up the meeting.
Conservatives warned that the meeting may involve the King in the UK-EU deal-making process or implicitly endorse it.
The European Commission deputy chief spokeswoman claimed the meeting was "not part of this effort" to strike a settlement.