In Depth

Brexit: What are the pros and cons of leaving the EU

The arguments for and against membership of the European Union

Security

Former work and pensions secretary Iain Duncan Smith, who was in favour of Brexit, said we were leaving the "door open" to terrorist attacks by remaining in the EU. "This open border does not allow us to check and control people," he said.

However, several senior military figures, including former chiefs of defence staff Lord Bramall and Jock Stirrup, argued the opposite.

In a letter released by No 10, they said the EU was an "increasingly important pillar of our security", especially at a time of instability in the Middle East and in the face of "resurgent Russian nationalism and aggression".

Defence Secretary Michael Fallon also said the UK benefitted from being part of the EU, as well as Nato and the United Nations. "It is through the EU that you exchange criminal records and passenger records and work together on counter-terrorism," he said. "We need the collective weight of the EU when you are dealing with Russian aggression or terrorism."

In contrast, Colonel Richard Kemp, writing in The Times said these "critical bilateral relationships" would persist regardless of membership, and that it was "absurd" to suggest that the EU would put its own citizens, or the UK's, at greater risk by reducing cooperation in the event of Brexit.

"By leaving, we will again be able to determine who does and does not enter the UK," said Kemp, a former head of the international terrorism team at the Cabinet Office. "Failure to do so significantly increases the terrorist threat here, endangers our people and is a betrayal of this country."

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