Real link between terrorism and Brexit made by David Cameron

They had not started counting the bodies after the attacks in Brussels on Tuesday when Ukip started making political hay and linking the tragedy to Britain’s referendum on EU membership.

Mike Hookem, the party's defence spokesman, was first out of the traps with a statement two hours after the bombs exploded.

Immediate suspension

“I’m once again calling for an immediate suspension of the Schengen agreement, the re-establishment of border controls and an end to the EU’s open door migration policies,” he said.

Party leader Nigel Farage followed suit later, retweeting a message by newspaper columnist Allison Pearson, saying: "Brussels, de facto capital of the EU, is also the jihadist capital of Europe. And the Remainers dare to say we're safer in the EU! #Brexit."

David Cameron said it was "not appropriate" to make a link between the attacks and the referendum and most commentators agreed Ukip had been crass and tasteless. But there is a link between the issue of terrorism and the referendum campaign, and the prime minister was first to make it.

In speech after speech in recent months, Cameron has cited security co-operation as one of the most important benefits of Britain's EU membership. He has explicitly cited co-operation on intelligence and the role of the European arrest warrant in accelerating the extradition of suspected terrorists.

“You know that we have that vital information when terrorists and criminals are travelling around Europe . . . The people who want to take a different path, they have to start answering some questions about what it would look like if we are not in that organisation and not party to those rules. And I know they fear that, that the time will come pretty soon when they have to start answering some of those questions,” he said.

Polls show fear is a big motivating factor for voters on both sides in the referendum, with those who favour staying in the EU fearful of the economic impact of leaving. Those who wish to leave are afraid of the impact of migration and Leave campaigners have been quick to link the terrorist threat in continental Europe to the Schengen open-borders regime.

The prime minister’s arguments about intelligence and judicial co-operation are persuasive to those who know about the international security bureaucracy. But many in Britain believe the country’s most important protection from terrorist attacks is the English Channel.

In a speech to the Politeia think tank in London on Tuesday night, former Conservative leader Michael Howard quoted a former head of Interpol who had said Schengen "is like hanging a sign welcoming terrorists to Europe".

Continuing relationship

Lord Howard, who wrote the speech before the

Brussels attacks

, said he simply disagreed with the prime minister about the security implications of leaving the EU.

"He thinks things like the European arrest warrant, our co-operation with Europol help to keep us safe. It doesn't follow that because we leave the European Union we have to leave Europol, that we have to dispense with the European arrest warrant. I think a continuing relationship of that kind would be perfectly achievable if we leave," he said.

Two polls published yesterday showed a fall in support for remaining in the EU. An ICM poll put the Leave side ahead, with 43 per cent to Remain’s 41 per cent. A ComRes poll gives Remain a lead of seven points but that is its smallest lead since the general election last year. Bookmakers have shortened their odds on Brexit since Tuesday’s attacks in Brussels, although they still expect the Remain side to win.