The U.K.’s future access to the European Union common market after it withdraws from the bloc will depend on whether it continues to keep its economy open to EU workers, according to the top Czech official in charge of EU affairs.

The Czech Republic and other EU members expect “tough negotiations” over new ties with Britain as the island nation prepares to exit the bloc, said Tomas Prouza, Czech State Secretary for EU Affairs. It will be very difficult for U.K. Prime Minister Theresa May’s new government to explain the “completely unrealistic” promises that politicians supporting Brexit made to voters before last-month’s referendum, he said.

Many of those who cast “Leave” votes did so in the hope that withdrawing from the EU will stem the flow of foreign workers into the country, particularly those from eastern European countries like Poland and Romania. That will not be possible if the country wants to continue enjoying unfettered access to the world’s largest trading bloc, he said.

“We are not willing to give up any of the four basic freedoms,” Prouza said. “If the British want free access to the EU market for its businesses; if they want free access for services, including financial; if they want free access for U.K. capital to the EU market, the freedom of movement of EU citizens must be preserved.”

There are about 1.5 million of eastern Europeans currently living the U.K.

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