Germany has warned that the EU’s border-free zone is in danger, after countries that didn’t have border controls before Schengen was set up reintroduced checks for the first time since the 1950s.

Sweden on Monday (4 January) imposed controls on travellers arriving from Denmark to try to curb an influx of refugees, prompting knock-on measures from Denmark that triggered fresh concern for Europe’s passport-free Schengen zone.

Hours after the Swedish controls on a major bridge-and-tunnel link went into effect, Denmark, which fears being saddled with large numbers of refugees, announced it too would implement spot checks on its border with Germany.

>>Read: Denmark introduces border controls with Germany

Alarmed by the restrictions, which come as both Germany and Sweden grapple with record migrant numbers, Berlin warned that Europe’s Schengen zone was “in danger”.

“Freedom of movement is an important principle – one of the biggest achievements (in the European Union) in recent years,” German foreign ministry spokesman Martin Schaefer told reporters.

The Schengen accord, considered a core achievement for the bloc, provides for borderless travel across most of the 28-nation EU based on the principle of free movement (see background).

“Schengen is very important but it is in danger,” he said.

Danish Prime Minister Lars Løkke Rasmussen cited the Swedish checks to justify his country’s immediate introduction of random border controls.

“We are simply reacting to a decision made in Sweden … This is not a happy moment at all,” he said.

Rasmussen warned Sweden’s controls could have a domino effect on Denmark, which received just 18,000 asylum requests in 2015, compared to Sweden’s estimated 160,000.

Brussels has granted Sweden a six-month exemption from the Schengen agreement on free circulation to allow it carry out ID checks on all travellers arriving from Denmark.

Extra security staff were on hand Monday at the Danish side of the Øresund crossing, a major entry point for migrants and refugees hoping to start a new life in Sweden.

The controls had proceeded smoothly by midday, but travellers were warned to expect longer queues and delays during the afternoon rush hour when commuters with jobs in Denmark return home to Sweden.

Under the new rules, rail passengers have to exit the train and clear checkpoints before boarding again – the first time in more than 50 years travellers between the two countries have been checked.

Those without valid ID will be refused entry. A private security company checked and photographed traveller IDs before allowing passengers on trains.

‘Building a Berlin Wall’

Officials at Danish train operator DSB confirmed they had seen a handful of people turned away, but would not specify if they were migrants or just commuters lacking proper ID.

“If they don’t have photo ID then we say sorry, we can’t let you on board,” DSB spokesman Tony Bispeskov said.

Rasmus Sandberg, a 25-year-old sergeant in the Danish military, was headed to Sweden for holiday and said he had not experienced any significant delay.

“I was afraid that it would take some time and that I wouldn’t reach the train but it was no problem,” he said.

The measures come after Sweden – which has taken in more asylum seekers per capita than any other European nation – said it could no longer cope with the unregulated flow of new arrivals.

A temporary fence has already been erected at Copenhagen airport’s Kastrup station, where trains will be stopped for mandatory controls.

“It’s as if we are building a Berlin Wall here. We are going several steps back in time,” said Michael Randropp, a spokesman for the local Kystbanen commuters’ association.

Several other European Union countries, including Germany, Austria and France, also re-imposed border checks last year as the continent grappled with its biggest refugee crisis since World War II.

More than one million migrants reached Europe in 2015, most of whom were refugees fleeing war and violence in Syria, Afghanistan and Iraq, according to the United Nations refugee agency.

Migrant flow reduced

In Sweden, the inflow has strained capacity to take care of the newcomers and authorities recently warned they would no longer be able to provide housing for everyone.

As the public mood soared, the government began reviewing its traditional open-door policy.

Some temporary border controls were already re-introduced on 12 November, after which the number of weekly arrivals dropped from a peak of 10,500 to some 3,500 by mid-December, according to Sweden’s Migration Agency.

In a further attempt to regain control over the flood of people arriving via Denmark, Sweden’s parliament last month passed a temporary law making transport companies responsible for ensuring that those arriving via the eight-kilometre (five-mile) Øresund crossing carry valid photo ID.

“I believe that these ID checks will be effective. More will have to seek asylum in other countries,” Migration Minister Morgan Johansson recently told the Swedish daily Dagens Nyheter.

Aside from the inconvenience the checks are expected to cause – especially for the roughly 8,600 daily commuters between Copenhagen and the southern Swedish city of Malmö – the new measures threaten to cause financial difficulties for Danish travel operators.

Commission President Jean-Claude Juncker has staked his leadership over Schengen, saying member states had to adopt controversial plans for a new border control force.

>> Read: Juncker stakes leadership on Schengen survival