BRUSSELS - European Union leaders formally agreed a Brexit deal on Sunday and urged Britons to back Prime Minister Theresa May's package in the face of furious opposition in the British parliament.

The 27 leaders took barely half an hour at a summit in Brussels to rubber-stamp a 600-page treaty setting the terms of Britain's withdrawal from the EU on March 29 and a 26-page declaration outlining a future free trading relationship.

May, who joined them shortly afterwards for what will be a brief meeting to seal the accord, now faces a struggle to get the deal, which has angered eurosceptics and EU supporters alike, through a deeply divided British parliament.

In an open letter to the nation, May said she would campaign "heart and soul" to get her Brexit deal through.

"It will be a deal that is in our national interest, one that works for our whole country and all of our people, whether you voted Leave or Remain," she said.

But any hope that she can clinch concessions to try to win over her critics was crushed by EU leaders.

Austrian Chancellor Sebastian Kurz, whose country holds the presidency of the EU until the end of the year, said: "One thing is clear. There will be no further negotiations."

European Union chief executive Jean-Claude Juncker said that while the deal was the best one possible, it was "no time for champagne or applause" following the agreement.

Meanwhile, European Council President Donald Tusk said the bloc was determined to have as close as possible a partnership with Britain, which has long been sceptical about deeper EU integration.

French President Emmanuel Macron said Britain's Brexit vote showed Europe needed reform. He stressed that Paris would hold Britain to tight EU regulations, notably on the environment, in return for giving it easy trade access.