Brussels: European Union President Donald Tusk warned the UK that it cannot expect frictionless trade outside the single market, lowering again Prime Minister Theresa May’s expectations as the pair prepare to meet in London.

“Friction is an inevitable side-effect of Brexit,” Tusk told an audience of business leaders in Brussels on Thursday.

With less than a month until a crucial summit when May wants to reach agreement on a post-Brexit transition period and start discussing a future trade deal, Tusk’s intervention is the clearest sign yet that the EU won’t compromise its main principles. The UK government has said it wants as little friction as possible in the post-Brexit trading relationship.

The draft of the final Brexit deal, published by the EU on Wednesday, angered the UK government, with May saying an option to keep Northern Ireland in the EU’s customs union and parts of the single market was unacceptable.

“Until now, no one has come up with anything wiser than that,” to prevent a hard border re-emerging between the two Irelands, Tusk said, adding that he would ask May to come up with her alternative during their meeting later on Thursday.

Ultimately, the UK’s relationship with the EU would be determined by the government’s “red lines,” he said, reiterating that with Britain ruling out continued freedom of movement and oversight by the European Court of Justice, it couldn’t expect close ties.

“We acknowledge these red lines without enthusiasm and without satisfaction, but we must treat them seriously, with all the possible consequences,” said Tusk, who’s drawing up guidelines for negotiations on the post-Brexit links to be unveiled next week. “Everyone must be aware that the UK red lines will also determine the shape of our future relationship,” he said.