European Union leaders have endorsed stiff divorce terms for Britain and warned Britons to have “no illusions” about swiftly securing a new relationship to keep their access to EU markets.

At a Brussels summit marked by unusual harmony among the 27 leaders, there was a flash of the cross-Channel acrimony which some fear could wreck any deal when officials accused London of cynically vetoing some EU spending and demanded it back down or face disrupting the start of talks next month.

Meeting for the first time since Prime Minister Theresa May triggered the two-year countdown to Brexit in late March, her counterparts took just minutes as they sat down to lunch in Brussels to approve eight pages of negotiating guidelines hammered out by their diplomats over the past month.

The text will bind Michel Barnier, their chief negotiator, to seek a deal that secures the rights of 3 million EU expats living in Britain, ensures London pays tens of billions of euros Brussels thinks it will be owed, and avoids destabilising peace by creating a hard EU-UK border between Northern Ireland and the Irish Republic.

With Barnier expecting to launch negotiations after the June 8 election May called to strengthen her position, the EU leaders also firmly ruled out discussing the free trade deal May wants until they see “significant progress” on key withdrawal terms.

“Before discussing the future, we have to sort out our past,” summit chair Donald Tusk told reporters.

He criticised British politicians for speaking of a quick deal to reassure expatriates when the legal complexities required much more detailed talks.

German Chancellor Angela Merkel repeated her concern that some in Britain had “illusions” about quick trade talks.

“I sometimes feel some people in Britain, and I don’t mean the government, are not so clear on the idea that there is an exit phase and then a phase on the future relationship,” she said.

European Commission President Jean-Claude Juncker, the EU chief executive, took an even more combative tone, saying his officials had identified a “cocktail” of 25 different legal issues to settle in regard to expatriate residence rights alone.

The EU considers it vital that Britain not be seen to profit from Brexit, to dissuade others from following suit.

In a mark of how last year’s Brexit vote has called into question the unity of the United Kingdom itself, leaders offered Irish Prime Minister Enda Kenny a pledge that if Northern Ireland, which voted against Brexit, ever unites with his country, it will automatically be in the EU.