EU Council president Donald Tusk insists Brexit vote has to be respected
The British Prime Minister is understood to have objected to attempts to sideline the UK almost as soon as her debut EU summit began.
Theresa May intervened at the start of the conference on Thursday afternoon to register her difficulty at the European Union trying to decide future policy in the absence of the UK.
She said the UK would refuse to "rubber-stamp" decisions made during meetings of the remaining 27 European Union nations - a reference to an "informal" summit held in Bratislava in September, the first without Britain for 43 years.
"Mrs May said that she will not accept rubber-stamping EU decisions if the UK is not invited," EU sources told Sky News, although it is understood she was not suggesting the meetings of the remaining countries should end altogether.
The Tory leader's immediate intervention took place during a presentation by Slovak prime minister Robert Fico.
It reflects concerns the UK is being sidelined even before Brexit takes place.
EU Council president Donald Tusk told Mrs May at the summit table that this was "all a result of the UK (Brexit vote) and that the EU 27 will have to keep meeting".
There is another EU 27 meeting scheduled for the end of January in Valletta, Malta.
A UK source confirmed the intervention, suggesting it would be inappropriate for the UK to enforce at a full EU summit of 28 an agenda that it had not discussed and was not a party to.
The UK will not object to EU-27 meetings, as long as "it's business for 27", it is understood.
The 'Bratislava Declaration' covered security, migration and youth unemployment.
It is unclear whether it will now feature in Friday's conclusions of the full European summit.
After the summit, Mr Tusk said: "After the decision in the UK, we have to respect the decision of the referendum.
"If it is reversible or not, this is in the British hands.
"I would be the happiest one if it was reversible but we now we have to start our formal works."