Does Britain’s membership of the EU makes us less safe?

That’s what Leave campaigners Iain Duncan Smith and Boris Johnson have suggested, among others.

Mr Duncan Smith said last month that a Paris-style attack is more likely to happen here if the UK votes to Remain in the EU.

And Boris Johnson wrote in a Telegraph article: “It is the border-free Europe, obviously, that makes it so much easier for our enemies to move around.”

But what are the facts?

Open borders

In an interview with the BBC, Mr Duncan Smith said: “I think the present status of the open border we have right now many of us feel does actually leave that door open and we need to see that resolved.”

Judging from the comments section on previous FactChecks, some people are confused by what this talk of “open borders” and “open doors” means.

It is not possible to enter Britain from the EU without having your travel documents checked, although that has been the norm when travelling between most countries in continental Europe until very recently.

That’s because Britain is not part of the Schengen Agreement, which lets people travel between 26 European countries without passport checks.

The Schengen area is not dependent on EU membership. Some non-members like Norway and Switzerland are in Schengen, while members Britain and Ireland stayed out.

Since the Paris terror attacks in November last year, Germany, Austria, Hungary, Denmark, Sweden and others temporarily reintroduced border controls, with non-EU travellers particularly likely to have their documents checked.

Eurosceptic politicians are not the only people to say that the Schengen rules make it easier for terrorists to move around the continent.

Boris quotes Ronald Noble, the former head of Interpol, who in the aftermath of the Paris attacks wrote that Schengen “is effectively an international passport-free zone for terrorists to execute attacks on the Continent and make their escape”.

What’s not clear is how voting for Britain either to Leave or Remain in the EU would affect any of this – the problem would still be there if Britain left the EU.

What Mr Duncan Smith appears to be saying is there is a danger of terrorists posing as asylum seekers, coming to Europe, eventually getting EU citizenship, then using the right to freedom of movement to stay in Britain more easily than would presumably be the case after Brexit.

He said: “Who is to say in the next few years that countries who have taken people from various areas aren’t then going to give them leave to remain, and even passports, as we’ve seen in some cases, and then in due course may well turn up again in the UK?”

“This open border does not allow us to check and control people that may come and spend time.”

Is this a realistic threat? It’s hard to say, although we can say that so far, the main danger to the UK has not come from people entering this country from Europe.

Home-grown terrorists

Sir Jonathan Evans, the former head of MI5, responded to Mr Duncan Smith by saying: “The terrorist threats to the UK in recent years, including many that have arisen within our own communities, have not been the result of EU border policy.”

This is borne out by the latest Home Office figures, which show that of the 507 people convicted of terror-related offences in the UK since 11 September, 2001, only 23 (4.5 per cent) were from EU countries, and eight of them were Irish.

This compares to 332 convicted terrorists – 65 per cent of the total – who were UK citizens.

Since 9/11, five men have died carrying out terror attacks on UK soil. One of them – Kafeel Ahmed, who died after trying to blow up Glasgow airport in 2007 – was Indian.

Three of the four men who killed 52 people in the attacks on London on 7 July 2005 were UK citizens. The fourth bomber, Germaine Lindsay, was born in Jamaica but had British residency.

Does the EU make it easier for countries to work together to fight terrorism?

Boris Johnson and Richard Walton, former head of counter-terror at Scotland Yard, say no.

They argue that Britain could still strike intelligence deals with foreign countries even if we left the EU, like the Five Eyes alliance we have with Australia, Canada, New Zealand and the US.

Mr Walton writes: “Membership of the EU does not really convey any benefits that we couldn’t access if we were outside it, with the single exception, perhaps, of the European Arrest Warrant – although this is probably of more use to those who deal with serious and organised crime, rather than terrorism.”

It’s true that the European Arrest Warrant is only rarely used by Britain to request the extradition of terror suspects. There were only six terror-related requests out of a total of 1,428 between 2009/10 and 2014/15.

But it was a European Arrest Warrant that saw 21/7 bomber Osman Hussain extradited from Italy back to Britain after he escaped in 2005.

The European Commission says the warrant system has cut the time it takes to extradite unwilling suspects from around 1 year to 48 days.

The verdict

It’s important to remember that Britain does not have an “open border” with Europe in the same way that the Schengen countries – until recently – had with each other.

What Iain Duncan Smith appears to be worried about is the prospect of extremists entering other EU countries as migrants and eventually getting citizenship, making it easier for them to live and work in Britain.

This is a prediction, so it’s impossible to prove him right or wrong.

The most we can do is look at recent history, which shows that the threat from within these borders has so far been much greater than terrorists coming here from Europe – but these figures pre-date the recent surge in migration.

As we have seen, experts are divided on the impact Brexit would have on the UK’s ability to protect itself from security threats.