BRUSSELS -- European Union chief Donald Tusk vowed Thursday not to compromise on EU principles in negotiating Britain's departure from the bloc and warned that London is heading for a hard exit.

"Our task will be to protect the interests of the EU as a whole and each of the 27 member states," Tusk said.

He insisted that Britain cannot hope both to stay in Europe's single market and restrict the movement of EU migrants, saying "there will be no compromises."

Tusk, who chairs talks among EU leaders as president of the European Council, said London had chosen "to radically loosen relations with the EU -- something that goes by the name of 'hard Brexit'."

Prime Minister Theresa May has said she will trigger Britain's exit negotiations before the end of March. May also appeared to signal that her government would prioritize controls on immigration over access to the European single market, an approach informally called a "hard Brexit."

Once May activates the exit clause -- Article 50 in the EU's governing Lisbon Treaty -- negotiations on the terms of Britain's departure would run for two years. The time frame could be extended, but only if the 27 remaining member states agree unanimously.

Still, few EU members want to see Britain go, and Tusk said London could withdraw its decision to leave during those two years.

"There are no legal barriers for this kind of decision, and I have no doubt this scenario will be acceptable for all European partners," he told experts and officials at a European Policy Centre event.

Britain's foreign minister, Boris Johnson, has said the U.K. government intends "to have its cake and eat it" by remaining in the EU's single market while limiting the movement of EU residents into the U.K.

Britain is assuming that European partners will compromise given that the country is the bloc's second-biggest economy. But Tusk asked his audience at the European Policy Centre think-tank event to try a simple experiment: "Buy a cake, eat it, and see if it is still there on the plate."

"The brutal truth is that Brexit will be a loss for all of us. There will be no cakes on the table, for anyone. There will be only salt and vinegar," Tusk said. "The only real alternative to a hard Brexit is no Brexit."

Pressure is mounting on May as the June exit referendum rattles the British economy and pound, while Scotland mulls a new independence vote, activists launch court challenges and lawmakers clamour for a say on the negotiating terms for Brexit.

Raf Casert in Brussels contributed.