Angela Merkel has stressed that Britain cannot have access to the EU's internal market after Brexit if it limits immigration from the bloc.
The German chancellor said that giving Britain full access to free trade across Europe's border but allowing it to restrict the movement of people would lead to a free-for-all.
"If we don't say that full access to the internal market is linked to full acceptance of the four fundamental freedoms, a process will spread in Europe in which everyone does what they want," she said.
She also told a group of German businesspeople that upholding the EU's "fundamental freedoms," including free movement of EU citizens, remained essential.
Theresa May has said she plans to formally trigger EU exit talks by the end of March.
The British prime minister told delegates at her party conference on Wednesday that she wants any agreement with the EU to "involve free trade, in goods and services".
She added: "I want it to give British companies the maximum freedom to trade with and operate within the single market - and let European businesses do the same here."
But she said: "Let's state one thing loud and clear: we are not leaving the European Union only to give up control of immigration all over again.
"We are leaving to become... a fully sovereign and independent country - and the deal is going to have to work for Britain."
Mrs Merkel told the German businesspeople: "They won't be easy negotiations."
EU member states, the European Commission, and the EU Parliament have stuck doggedly to the concept of "no notification, no negotiation", before Article 50 is triggered.
In preparation for parrying any attempts by the UK to win concessions, diplomats in Berlin have already carried out an exercise mimicking the negotiations.
Beyond that, other countries have begun putting forward starting positions that could be used in exchange for approving British requests - such as Spain proposing shared sovereignty of Gibraltar.
The Irish government will host an ill-island forum on Brexit next month.
