Leading article: Europe is in danger of eroding one of its greatest achievements

In 1951 the Foreign Secretary, Ernest Bevin, was asked about his vision of a British foreign policy. He replied: "My policy is to be able to take a ticket at Victoria Station and go anywhere I damn well please".

It was an unlikely dream at the time. The Iron Curtain had descended across the continent. A Europe in which people could travel unimpeded by the need to produce passports, papers and visas seemed a utopian notion. Yet Bevin's dream of free movement across the Continent has been realised.

Since 1995, 25 European states (sadly not Britain) have signed the Schengen Agreement, which abolishes internal EU borders. European citizens can now travel from Calais to Bari, from Cádiz to Helsinki, without producing a passport at border posts. There are fewer more potent manifestations of European unity.

But it is an achievement that is under threat. European Union home affairs ministers meeting in Brussels this week agreed on a new "mechanism" to allow border controls to be temporarily re-imposed. This is a response to pressure from the Italian and French governments who were embroiled in a diplomatic spat last month when the French authorities prevented a trainload of Tunisian migrants from travelling across the border. The plan now needs to be approved at next month's EU leaders' summit. It will also need to be sanctioned by the European Parliament.

The European Commission says border closures must be a last resort and are to be used only in exceptional circumstances. But there is a danger that, if the principle of free movement is eroded, closures will become routine. It is unclear as to what will trigger border closures. And this gives scope for governments to act unilaterally. There are already signs of that happening anyway. Denmark announced this week that it will reinstate control booths on its German and Swedish borders, despite the fact that there is no indication of North African migrants heading to Denmark. Copenhagen was responding to pressure from the anti-immigration Danish People's Party. If other governments give in to xenophobic domestic elements in this way Schengen could rapidly unravel.

The recent surge in migrants from North Africa does necessitate an active response from the European Union. Italy and Malta, the two states closest to the crisis zone in the Maghreb, have every right to request assistance from other member states to deal with the 25,000 desperate individuals who have made the sea journey this year from Tunisia and Libya. Member states should also share the upfront costs of accommodating these refugees. A common asylum system is a sound idea (and it is depressing that our own Government has been leading the opposition to it). But making it easier for EU member states to close their borders is the worst possible response.

The Arab uprisings should be a moment of optimism for Europe. Populations across the Muslim world are challenging repressive rulers and demanding their democratic rights. But Europe is responding to the instability and disruption caused by this great revolution with anxiety and petty self-interest. Worse, Europe's leaders are chipping away at one of the achievements – freedom of movement – that have made this continent such a beacon of hope to oppressed populations around the world.