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"The two questions of freedom of movement for people and freedom of movement for commerce are not necessarily related," said Camino Mortera-Martinez, a research fellow at the Centre for European Reform. "But the closing of the border would create enormous obstacles to trains and trucks."

Charles Grant, director of the CER, says the emergency brake is fraught with difficulties. "British officials want to be able to use the brake immediately. Yet the mechanism could well require EU legislation, which often takes more than a year. Second, according to what criteria would use of the brake be permissible? The British will want the criteria to be vague enough for them to be able to pull it easily; the commission and other member states will try to make the criteria objective and hard to satisfy."

Charles Grant, director of the CER, said the deal, on the face of it, had the makings of a workable offer for Mr Cameron, but that its worth would depend on the details of any agreement."Given the current state of the negotiations, the British side would probably be happy to accept a deal that includes a four-year 'emergency brake' on benefits," he said.

„De handelsbelangen zijn te groot om helemaal terug naar af te gaan”, zegt Rem Korteweg van het in Londen gevestigde CER. ...„Stel: Schengen stort in. Wat zijn de gevolgen voor de interne markt? Je krijgt verwijten over en weer, de sneeuwbal wordt een lawine. ‘Niemand wilde dat het gebeurde, het was ook absurd, maar het gebeurde toch’ – de geschiedenis staat er bol van.”

But the Centre for European Reform argues that Europe should not dismantle Schengen – but rather improve the system by processing refugees more effectively, boosting external border security and sharing intelligence. It also suggests that the fake Syrian passport in Paris could have been a deliberate plot to reignite the debate about Europe's asylum policies.

Britain Stronger in Europe said one piece of research showed UK goods trade with the EU was 55% higher because of EU membership. The research, from the Centre for European Reform, suggested the effect of EU membership in 2014 was worth £133bn to 200,000 British firms that import and export.

Britain Stronger In Europe highlighted research by the Centre for European Reform which found found that Britain's goods trade with the EU is 55 per cent higher as a result of its membership.The "EU effect" was worth around £133 billion to the 200,000 export and import companies in the UK in 2014, it said.

"Estamos en un impasse porque es invierno y se ha reducido la llegada de refugiados, pero en un par de meses la presión volverá a aumentar y, si no se han tomado medidas, es posible que se produzca un efecto dominó de cierre de fronteras que acabe con Schengen", explica a EL ESPAÑOL la analista del think tank CER, Camino Mortera-Martínez, que cifra en un 51% las posibilidades de que se materialice este escenario.