Senior cabinet minister Iain Duncan Smith has been accused of "irrelevant scaremongering" by counter-terrorism experts over claims Britain would be more vulnerable to Paris-style terror attacks if the country stayed in the European Union.
On Sunday, the work and pensions secretary, who is backing the campaign for Britain to leave the EU, warned that the union's freedom of movement leaves the "door open" for a similar incident in the UK.
This claim is likely to be repeated in the months leading up to the referendum on June 23.
However, security experts have accused Duncan Smith of "conflating" different issues. Raffaello Pantucci, the director of international security studies at the Royal United Services Institute, told BuzzFeed News: "The reality is that it is not the case that we would be any more exposed to the risk if we were in the union or not."
A spokesperson for Vote Leave directed BuzzFeed News to a briefing on security issues on the anti-EU group's website, which says that EU rules mean there are fewer "systematic checks" on people who have EU passports entering the UK.
"This makes it much easier for terrorists fighting abroad to return to Britain, who need not pass through a single border control between arriving on the shores of Greece and reaching the English Channel," the report noted.
"The reality is that it is not the case that we would be any more exposed to the risk if we were in the union or not."
While Pantucci admitted the system is more "lax" when it comes to the movement of EU citizens, he warned that pulling out of the union could make the work of border officials more difficult.
He also claimed that, contrary to Duncan Smith's remarks, leaving the EU might lead to member states withholding intelligence from the UK.
"To address the current Europe-wide threat that is faced, there is a growing approach within the union to seek ways of cooperating more closely, and developing joint centres (like the SitCen) to share intelligence and information about travellers," Pantucci said.
"It is hard to see how the UK’s life would be made easier in trying to tap into this intelligence integration if it was sitting outside the union."
A former head of counter-terrorism with the City of London police called the cabinet member’s comments "irrelevant scaremongering by ignorant people, who should take the time to get better informed".
Kevin Hurley, now a police and crime commissioner in Surrey, told BuzzFeed News the arguments about security from anti-EU campaigners were irrelevant. "As an expert of security and immigration, they sicken me with their political gesturing and complete ignorance," he said.
Hurley has not yet decided how he will vote in the referendum but said the risk of terrorism would not change regardless of the outcome of the vote.
He said there are two main concerns when it comes to security: first, the main terrorist threat to the UK is coming from people radicalised in the UK; second, cuts to the border force meant returning jihadis might slip under the radar and funding would need to increase whether Britain left the EU or voted to stay in.
"Frankly, it's irrelevant scaremongering," he said.