Schengen: Europe's passport-free travel zone 'on brink of collapse'
The system that allows passport-free movement across Europe is feared to be on the brink of collapse as nations tighten their border controls because of the refugee crisis.
European Council president Donald Tusk has admitted it is a "race against time" to save the Schengen travel zone, which has operated for 30 years.
The UK is among only six of the 28 EU member states that sit outside the zone.
ITV News Political Correspondent Romilly Weeks reports:
How is it under threat?
More and more participating nations have taken action to face the influx of refugees - who entered the EU without Schengen visas - at their border.
The EU's border control agency Frontex has said a record 1.2 million illegal crossings were made at the EU's external borders in the first 10 months of this year.
Sweden has temporarily reimposed passport checks at the border and Germany has introduced new measures, while Slovenia has erected barbed wire frontier fences.
Austria and Hungary have already taken measures to control migrant entries, while the Netherlands has reintroduced spot checks.
If those measures were to become permanent it would clearly undermine the principle of freedom of movement upon which the Schengen zone was established.
What can save Schengen?
“Saving Schengen is a race against time and we are determined to win that race," European Council president Donald Tusk has said after a migration summit with African leaders in the Maltese capital Valetta.
"This includes, first and foremost, restoring external border control," Mr Tusk added. "Without effective control on our external borders, the Schengen rules will not survive."
The council president spoke before another informal summit in Valetta dedicated to the issue of Schengen and urged: "We must hurry, but without panic."
When was the Schengen established?
Signed in 1985 in the Luxembourg town of Schengen, the agreement of 26 countries allows anyone to move freely between the member stages without showing a passport or visa.
Those outside the EU are free to move unchecked between the countries once they have been checked for ID upon initial entry.
A Schengen visa - which lasts 90 days - costs up to £44.
The system effectively bans permanent passport checks across the zone, though police can perform targeted checks at borders while participating states can tighten controls in an emergency.
All of the countries that have introduced border control measures amid the refugee crisis have declared them temporary so as not to breach the 30-year-old accord.
Where does Britain stand on the issue?
Though Britain has always remained outside Schengen, Prime Minister David Cameron suggested the participating nations needed to take action as the continent looks to deal with the movement of refugees.
"Clearly you need to have either a system with external borders or a system with internal borders," Mr Cameron said. "You can't have borders that don't work at either level."
He said Britain has "done more than any other European country to support the European asylum support officers" on Schengen's external borders.