BRUSSELS — European Union leaders vowed Saturday to stand shoulder-to-shoulder behind their negotiating team during the divorce proceedings with Britain and warned that demands from British Prime Minister Theresa May will be dealt with “firmly.”

The 27 EU leaders in Brussels finalized the cornerstones of their negotiating stance within four minutes of starting a short smooth summit, a month after the British leader triggered two years of exit talks on March 29. The negotiations themselves are to open shortly after Britain holds an early election on June 8.

“We now have unanimous support from all the 27 member states and the EU institutions, giving us a strong political mandate for these negotiations” under chief negotiator Michel Barnier, EU Council President Donald Tusk said.

Tusk said there can’t be any discussions on the future relationship between the EU and Britain until there has been major headway on key issues.

“We must first achieve sufficient progress on citizens’ rights, finances and the border issue in Ireland. It is too early to speculate on when this might happen,” Tusk said Saturday.

He said the 27 leaders would unanimously have to say there was “sufficient progress” to allow the talks to go to the next phase. That would give any EU country with a dispute with Britain, like Spain over Gibraltar, major influence over the timetable of the talks.

The negotiating guidelines also halted British hopes of having future trade relations being discussed concurrently through the talks.

“Before discussing the future, we have to sort out our past. We will handle it with genuine care – but firmly,” Tusk said.

Some at the summit were already considering how to deal with possible British negotiating tactics.

“Maybe the British government will do its utmost to split the 27 nations. It is a trap we need to avoid,” said Belgian Prime Minister Charles Michel.

Ever since the June 23 referendum last year in which Britons narrowly voted to leave the bloc, the unity of the remaining 27 EU nations “has been really exemplary,” said German Chancellor Angela Merkel.

In contrast, citizens in Britain have been divided because of the momentous changes looming.

The EU is also intent on making Britain pay a divorce bill, which some EU officials have put as high as $65 billion.

The money aims to pay for everything from pensions to financial commitments already made in the EU’s 7-year-budget, which runs until 2020.

French President Francois Hollande said the leaders agreed on “a simple principle,” applicable to Britain or any other country that might want to quit the bloc in the future, “that they must not be in a more favorable situation on the outside than they were on the inside.”

“There is always a price, a cost, a consequence from quitting the Union,” Hollande said at his farewell European summit.

To kick off the negotiations with Britain, Tusk wants to center on the millions of people living in each other’s nations 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.