Germany no longer rules out a British withdrawal from the European Union, weekly Der Spiegel reported on Sunday. British prime minister David Cameron’s proposal to limit free labor migration in Europe — one of the union’s cornerstone integration policies — would be a bridge too far for his German counterpart, Angela Merkel.

Citing unnamed government sources, Der Spiegel said a “point of no return” could be reached in Anglo-German relations.

Earlier, Merkel had seemed appreciate of Cameron’s push to renegotiate his country’s membership terms of the European Union, telling lawmakers in London that while she could not “pave the way for a fundamental reform of the European architecture which will satisfy all kinds of alleged or actual British wishes,” Germany was still prepared “to pay almost any price to keep Britain in the European Union.”

If Der Spiegel‘s reporting it accurate — and a Downing Street official would not dispute it to The Guardian newspaper — that clearly does not include tinkering with one of the European Union’s “four freedoms,” which also include the free movement of capital, goods and services.

Cameron has proposed scaling back labor migration in what is generally interpreted as an attempt on his part of woo back voters from the United Kingdom Independence Party which wants Britain to leave the European Union altogether. Recent polls put the nationalist party’s support between 15 and 18 percent. If that support holds up in May’s general election, it could deny Cameron’s Conservatives a plurality of the seats in the next parliament.

The prime minister has not spelled out in much more detail what changes in Britain’s relationship with the European Union he seeks but promised voters a referendum on the country’s future membership if he is reelected next year.

Cameron and Merkel do share priorities in Europe, including deepening the internal market and reining in excessive government spending and debt, but their relations have been rocky.

Earlier this year, Cameron voiced strong opposition to the appointment of former Luxembourg premier Jean-Claude Juncker, a European federalist, to head the incoming European Commission. Merkel supported him.

In 2011, the British leader vetoed a proposal for closer economic and fiscal integration in Europe. Under German leadership, the countries that use the euro went ahead with such reforms anyway, effectively creating a “two tier Europe” with an integrated eurozone core and peripheral states resisting federalization but participating in the single market.

As Europe’s third largest economy after Germany and France, liberal Britain can act as a counterweight to the protectionist instincts of many continental nations and be a German ally. But Germany also relies heavily on trade with its neighbors. Limiting the free movement of people in Europe could harm its economy and erode the European project as a whole.