Theresa May has been given a stark warning that her Brexit red lines will inevitably cause friction in trade between the UK and EU.
The Prime Minister is facing a frenetic 48 hours of Brexit diplomacy as she tries to secure a united Cabinet ahead of crunch talks with European Council president Donald Tusk and a keynote address on the UK's future relationship with the EU.
But ahead of their Downing Street meeting, Mr Tusk delivered his assessment of the possible post-Brexit trading relationship and defended the EU's position in the row over the Irish border.
UK red lines will determine shape of our future relationship. Next week I will present draft guidelines on this. Let's be clear: there can be no frictionless trade outside the customs union and Single Market. Friction is an inevitable side effect of #Brexit. By nature.
-- Donald Tusk (@eucopresident) March 1, 2018
Mrs May strongly rejected parts of a draft legal text from the European Commission regarding the post-Brexit status of Northern Ireland, which she said would "threaten the constitutional integrity of the UK" by creating a border down the Irish Sea.
But in a speech in Brussels, Mr Tusk said he was "absolutely sure that all the essential elements of the draft" would be accepted by the 27 remaining EU members and stressed that the EU's chief negotiator, Michel Barnier, had the support of the bloc's leaders.
Ahead of his talks with Mrs May, Mr Tusk said the UK Government's decision to rule out membership of the single market and customs union had been acknowledged "without enthusiasm and without satisfaction".
"One of the possible negative consequences of this kind of Brexit is a hard border on the island of Ireland.
"The EU wants to prevent this scenario and, if no other solution is found, the proposal (is) to establish a common regulatory area comprising the Union and the UK in respect of Norther Ireland.
"And, until now, no-one has come up with anything wiser than that. In a few hours I will be asking in London whether the UK Government has a better idea that would be as effective in preventing a hard border between Ireland and Northern Ireland."
The red lines will also determine the shape of the future UK-EU relationship, Mr Tusk said.
The Prime Minister has said she wants a deal which will allow trade to be "as frictionless as possible".
But Mr Tusk warned: "There can be no frictionless trade outside of the customs union and the single market.
"Friction is an inevitable side-effect of Brexit by nature."
Mrs May will set out further details of her Brexit plans in a major speech on Friday.
Ahead of that she has called the gathering of her top ministerial team on Thursday to discuss the "end state" withdrawal deal Britain wants to hammer out with Brussels.