BRUSSELS • British Prime Minister Theresa May's first European Union summit since her disastrous election could mark the scene of the first big row of the Brexit negotiations as she outlines how Britain proposes to treat the bloc's citizens after it leaves.

Mrs May was due to run her fellow leaders through the principles of her approach towards EU nationals over dinner in Brussels yesterday before publishing a detailed plan next Monday.

While this week's opening round of Brexit negotiations was considered by the two sides to be generally constructive, diplomats said Mrs May's plan for expats could lay bare the divisions between them.

"It's an early test," said Mr Jonathan Portes, professor of economics and public policy at King's College London. "If we can't get a sellable deal on this issue, which is less problematic than others, then we might be in trouble."

Britain's stance on the rights of an estimated 4.5 million European and British nationals living in each other's countries is expected to fall short of the EU's expectations, officials in Brussels said.

Failure to find common ground quickly would jeopardise Britain's ability to strike a Brexit deal by the time it leaves the bloc in March 2019 and would likely at the very least delay when EU governments would be willing to discuss the trade accord Mrs May wants.

While British and EU officials say they want to end the uncertainty for the millions of people involved, there are obstacles. They include what the rights are, which citizens get to enjoy them and whether Europe's top court will have the power to enforce them.

Mrs May must find a middle way between satisfying EU governments without running into opposition from euro-sceptics at home.

The EU is demanding that European nationals who move to Britain before it leaves the bloc keep the same rights as they currently have throughout the continent. This could give them higher protection than those enjoyed by British citizens, something that Britain is likely to say is unacceptable, EU officials said.

Central to this is the right to bring a non-EU spouse to Britain, which, when she was Home Secretary, Mrs May made more difficult for Britons. European citizens face far fewer hurdles and the EU is demanding that this continues. While Mrs May has insisted there will be no turning back from Brexit, a number of EU leaders have said in the past week that the "door was open" for Britain to remain in the EU.

In the latest comments, EU president Donald Tusk yesterday channelled former Beatle John Lennon to suggest it was not too late to change tack.

"Some of my British friends have asked me whether Brexit could be reversed, and whether I could imagine an outcome where the UK stays part of the European Union," Mr Tusk told reporters.

"You may say I am a dreamer, but I am not the only one," he added with a broad smile, quoting Lennon's iconic song Imagine.

BLOOMBERG, AGENCE FRANCE-PRESSE