European Union leaders have warned that any demands from UK Prime Minister Theresa May during divorce proceedings with the EU will be dealt with "firmly".
The 27 EU leaders in Brussels finalised the cornerstones of their negotiating stance without much trouble during a short summit, a month after May triggered two years of exit talks on March 29.
The negotiations themselves are to start shortly after snap elections in Britain on June 8.
"Guidelines adopted unanimously. EU27 firm and fair political mandate for the #Brexit talks is ready," EU Council President Donald Tusk tweeted on Saturday.
He insisted that there can be no discussions on a future relationship before some key issues like how much Britain owes the bloc are sufficiently agreed, and he said that the welfare of citizens and families living in each other's nations would be the priority once the talks start.
It countered British hopes to have future trade relations running in sync all through the talks.
Tusk said however that "before discussing the future, we have sort out our past and we handle it with genuine care - but firmly."
And some were already considering how to deal with British tactics.
"Maybe the British government will do its utmost to split the 27 nations and it is trap we need to avoid," said Belgian Prime Minister Charles Michel.
Watch: EU Council President Tusk takes firm line on Brexit
Ever since the June 23, 2016, referendum decided the departure of Britain from the bloc, the 27 have shown rare unity.
Now, the EU is also bent on making Britain pay the divorce bill, which some EU officials have put as high as 60 billion euros ($A87 billion).
"If you are no longer part of a club, it has consequences. A Brexit for free is not possible," Michel said.
To kick off the negotiations, Tusk wants to centre on the millions of people living on each other's side who would be immediately affected.
All sides "need solid guarantees for all citizens and their families who will be affected by Brexit on both sides. This must be the No. 1 priority," Tusk said.
Some three million citizens from the 27 nations live in Britain while up to two million Britons live on the continent, all facing uncertainly on such issues as welfare and employment.
Watch: Merkel expects demanding Brexit negotiations
