Germany’s decision to tighten controls at every one of its land borders seems driven chiefly by politics, is difficult to justify in law, deals a heavy blow to Europe’s prized free movement and could severely test EU unity.

Berlin said on Monday that controls in place at its border with Austria since 2015, and since last year with Poland, the Czech Republic and Switzerland, would be extended next week to France, Luxembourg, Belgium, the Netherlands and Denmark.

The move would curb migration and “protect against the acute dangers posed by Islamist terrorism and serious crime,” said Nancy Faeser, the interior minister.

The most recent in a series of deadly knife attacks in which the suspects were asylum seekers, in Solingen last month, came days before crunch regional elections in eastern Germany that resulted in the far-right, anti-immigration Alternative for Germany (AfD) party scoring historic successes in two states.

Polls show migration is also voters’ biggest concern in Brandenburg, which holds its own elections in a fortnight – with Olaf Scholz’s centre-left Social Democratic party forecast to finish behind the far-right party – and the chancellor’s ailing coalition seems to be heading toward a crushing defeat in federal elections next year.

“The intention of the government seems to be to show symbolically to Germans and to potential migrants that the latter are no longer wanted here,” said Marcus Engler of the German Centre for Integration and Migration Research.

Faeser said the new controls would include a scheme allowing more people to be turned back directly at the border, but declined to go into detail. Officials and diplomats in Brussels have expressed dismay, calling the move “transparent” and “obviously aimed at a domestic audience”.

Germany’s central position in the EU and its status as the bloc’s largest economy, mean the controls, due to come into force on 16 September for an initial six months, could have an impact that reaches far beyond the country’s voters.

In principle, Europe’s passport-free Schengen area, which was created in 1985 and now includes 25 of the 27 EU member states plus four others including Switzerland and Norway, allows free movement between them all without border controls.

Temporary checks are allowed in emergencies and exceptional circumstances to avert specific threats to internal security or public policy, and have typically been imposed after terror attacks, for major sports events and during the pandemic.

Increasingly, however, European governments, often under pressure from far-right rhetoric on immigration, have reimposed checks without the justification of concrete and specific threats, or clear arguments as to how controls can help mitigate them.

Although immigration policies and asylum follow-up procedures, for example, are decided nationally, European free movement, many observers argue, makes for an easy target – and “taking back control of borders” for effective headlines.

Besides Germany, Schengen members currently operating controls on particular borders include Austria, which cites Ukraine-related security threats and pressure on asylum to check arrivals from Slovakia, the Czech Republic, Slovenia and Hungary.

Denmark, citing terror threats related to the war in Gaza and Russian espionage risks, is carrying out checks on land and sea transit from Germany, and France is checking Schengen zone arrivals on the grounds of an increased terror threat.

Italy, Norway, Sweden, Slovenia and Finland are also operating border checks, variously citing terrorist activity, the wars in Ukraine and the Middle East, Russian intelligence activity, increased migration flows and organised crime in the Balkans.

As the guarantor of the Schengen treaty, the European Commission – which was notified of Germany’s plans on Monday – has generally accepted member states’ justifications for reintroducing temporary controls without demur.

Observers expect it to do the same for Berlin’s demand, despite the fact that there appears to be little clear practical justification – beyond an electoral threat from the anti-immigration far right – for checks on all nine of the country’s borders.

The commission said on Tuesday that member states were allowed to take such a step to address “a serious threat”, but the measures needed to be “necessary and proportionate” and must “remain strictly exceptional”.

The temporary German controls “represent a manifestly disproportionate breach of the principle of free movement within the Schengen area,” said Alberto Alemanno, a professor of European law at HEC Paris.

“It won’t fly under EU law – yet will this dissuade Scholz from going ahead?” he said. Christopher Wratil of the University of Vienna was even more scathing, accusing Berlin of “governing as if the AfD were [already] in power”.

After today, Wratil said, German politicians “should no longer tell me that somebody else is failing to comply with EU law … Wanting to wipe out Schengen with a mere stroke of the pen – and entirely without thinking.”

Others noted the economic value of the Schengen zone. A report by the Bertelsmann Foundation as long ago as 2016 estimated the reintroduction of internal border controls would cost Europe about €470bn (£397bn) in lost growth over 10 years.

Gerald Knaus, the chair of the European Stability Initiative think tank, also questioned the measure’s efficacy. “Internal border controls that are intended to have any effect mean the end of Schengen,” Knaus said on X.

They would also require “federal border protection and fences around Germany” and, moreover, “will fail if neighbours are not interested in participating,” he said.

After the EU finally agreed a hard-fought overhaul to its asylum and migration laws earlier this year, with the rules only due to come into force in 2026, European unity could be seriously tested if Germany asks its neighbours to take back large numbers.

Austria has already said it will refuse to take back any migrants refused at the German border, while the Polish prime minister, Donald Tusk, on Tuesday called Berlin’s decision “unacceptable” and demanded urgent consultations.