German Chancellor Angela Merkel issued a blunt warning to the United Kingdom Saturday, threatening the U.K. must grant European Union citizens unfettered rights to open immigration if the nation wishes to maintain access to the EU’s internal market.

Although Merkel has said that Germany wishes to maintain friendly relations with Britain in the Brexit aftermath, the German chancellor said the U.K. can only enjoy the EU’s market if it adheres to the union’s open borders immigration policies.

 The British people must “decide for ourselves how we control immigration” and be “free to pass our own laws.”

“I will always weigh things in Britain’s favor, but always with the thought that we 27 member states want to preserve our Europe,” Merkel said, adding, “we cannot work toward ultimately putting into question the whole European Union,” according to Fox Business.

Concern over unrestricted immigration, both the free flow of EU immigrants and refugees into the U.K., was seen as a key issue in the British voters’ decision to exit the European Union.

Britain has not yet triggered its formal Brexit procedure for leaving the EU, although Prime Minister Theresa May has indicated that the U.K. will begin the official process by the end of March. May has spoken to the immigration concerns of many Britons, saying the British people must “decide for ourselves how we control immigration” and be “free to pass our own laws.”

[lz_related_box id=”223566″]

Merkel stressed in her warning Saturday the U.K. would need to adhere to the EU bloc’s four basic principles in order to avoid possible backlash from member nations like Germany: freedom of goods, services, capital and people.

“If we don’t say that full access to the internal market is connected to complete acceptance of the four basic principles, then a process will unfold in Europe where everyone does and is allowed to do what they want,” Merkel said in early October while speaking to German business leaders, according to the Wall Street Journal. “That would be extraordinarily complicated.”