Freedom to pass across the borders of many European Union (EU) nations is enshrined in the Schengen Agreement but that licence for unrestrained passage now appears increasingly under threat.


In the face of relentless pressure to accept asylum seekers, two member nations have in recent days reintroduced border controls to more closely monitor their arrival.


Sweden has begun checking the travel documents of people arriving from Denmark across the 16km-long Oresund crossing, the tunnel and bridge link which connects Malmo in southern Sweden with Denmark’s capital, Copenhagen. In response, Denmark has now reinstated spot checks of those wanting to cross its southern border with Germany.


Denmark is in a difficult position, sandwiched between Germany, the destination for many Middle Eastern refugees, and Sweden, which last year had 163,000 asylum seekers, the most per capita of any of the 28 EU countries. Danish Prime Minister Lars Loekke Rasmussen is concerned that without checks on the German border, migrants and refugees might end up stranded in Denmark, unable to cross into Sweden.


The Schengen Agreement, which has scrapped passport control at common frontiers, is a crucial strand among the complex web of arrangements which keep the EU together. Twenty-two member states are part of the deal, which Britain and Ireland have opted out of, as are four non-EU members of the European Free Trade Association – Norway, Switzerland, Liechtenstein and Iceland.


Like toppling dominoes, the latest move by Sweden and Denmark puts a great deal of pressure on the pact, the integrity of which has already been threatened by several other countries – including Austria, Germany, Hungary and Slovenia – reintroducting controls last year on their Schengen borders in the face of the mass arrival of migrants.


Before the Oresund crossing was opened in 2000, the narrow and often stormy sound meant the northern Scandinavian countries – Norway and Sweden – were relatively isolated, insulated from Denmark across the water and from the rest of mainland Europe and its issues.


When the bridge and tunnel link was being built in the 1990s, one of the biggest concerns of the Swedish was it would allow wild animals, including dogs, raccoons and bats, to enter their country and introduce rabies. The British had a similarly unfounded fear when the Channel Tunnel to France was under construction.


The Oresund crossing is now a vital European link. While there are still language and currency differences on each side, about 18,000 people a day travel between the two countries, mostly from Swedish towns to the much larger Copenhagen, for work and study. The region the crossing serves accounts for more than a quarter of the GDP of both countries combined.


Initially it appeared Sweden might completely close the connection as a way of stemming the influx of migrants. That idea was met with widespread consternation by those whose livelihoods now depend on it.


When the link was opened, few would have predicted events in the Middle East might force consideration of its temporary closure. That reflects just how fast Europe is changing.


While each EU nation has the sovereign right to control its borders, Schengen has been one of the EU’s great successes. Just ask millions of travellers. It would be a shame to see these borders closing more permanently.