Germany has warned that the European passport-free Schengen Area is "in danger" after both Denmark and Sweden introduced new border controls on travellers within the zone.
"Schengen is very important but it is in danger," foreign ministry spokesman Martin Schaefer said, according to AFP news agency.
"Freedom of movement is an important principle -- one of the biggest achievements [in the European Union] in recent years," he is said to have added.
Reports of warning came after two Denmark's prime minister apparently announced it was introducing new controls on the border with Germany.
Sweden today also announced it was bringing in new ID checks on people entering from Denmark in a bid to calm an influx of migrants. From today, travellers will be stopped for mandatory checks on the border for the first time since the 1950s.
The Danish prime Minister Lars Loekke Rasmussen the new border controls came into force at noon today and were intended to be temporary, according to a report in Bloomberg.
He apparently said he was forced to act in response to Sweden's new checks, which he described as a “major step backwards" which would create “difficulty and problems" for many commuters between the countries.
The developments threaten to undermine Europe's 26-country Schengen area, which allows free travel without passports between member states.
The Schengen Agreement was first signed between five EU member states in 1985 and now comprises an area of 4,312,099 square kilometres with a population of more than 400 million people.