Visit nearly any big construction site in France, and chances are that many employees hail from low-wage East European countries. In Britain, farms employ labor from Poland, Romania or Bulgaria when the harvest comes. Nearly half of the drivers of trucks coming in and out of Spain are from nations to the east.

The principle underpinning all of that — the freedom of citizens from European Union countries to work anywhere in the 28-nation bloc — is a pillar of the union itself. In theory, it allows workers to move across the region to find employment and benefits businesses by providing a wider talent pool.

But companies have also long profited from rules that allow them to “post” workers from one country to another. Now, a backlash is growing across northern Europe amid increasing evidence that employers are taking advantage of the rules to hire low-wage foreigners rather than local citizens.

The French president, Emmanuel Macron, who promised to protect his compatriots from “unfair competition” from the east, is moving aggressively to focus attention on these posted workers as he begins a three-day tour of Central and Eastern Europe on Wednesday.