BRUSSELS — European Union leaders drew a stark line along the British Channel on Wednesday, telling the U.K. that it cannot keep valuable business links with its former continental partners in a seamless single EU market, if it doesn’t also accept European workers.

The challenge cuts to the heart of the British vote to leave the bloc following a virulent campaign where migration from poorer EU countries was a key concern. It also sets the scene for the complex departure negotiations facing departing Prime Minister David Cameron’s successor, for which nominations opened in London on Wednesday.

Meeting for the first time without the U.K., the 27 other EU nations set out a united strategy to face the next British government which will seek to salvage as many of the EU rights as possible while reneging on a maximum amount of obligations.

They emerged from the summit insisting that the “four freedoms” central to European unity are indivisible: the free movement of people, services, goods and finances.

The most palpable remaining link to Britain at the summit was the English language used. The remaining presidents, chancellors and prime ministers showed a firm common resolve, committing to be “absolutely determined to remain united,” EU Council President Donald Tusk said.

The leaders sought to dispel any notion that the referendum result will amount to their Waterloo.

“With a disunited United Kingdom, we need a united Europe more than ever,” Luxembourg Prime Minister Xavier Bettel said.

Tusk convened a special EU summit on Sept. 16 in Slovakia’s capital Bratislava to work out a plan to keep the EU united.