Staying a member of the European Union will make the United Kingdom more vulnerable to terrorist attacks, Work and Pensions secretary Iain Duncan Smith has warned.
Duncan Smith said that a lack of control on the UK's borders is a critical issue, and that Paris-style attacks could be more likely to take place if the UK votes to stay in the EU.
"This open border does not allow us to check and control people who may come and spend time here," he told the BBC. "We've seen what happened in Paris where they spent ages planning and plotting so who's to say it's not beyond the wit of man that those might already be thinking about that."
The Work and Pension secretary's comments come after Prime Minister David Cameron wrapped up his negotiations in Brussels and chaired a cabinet meeting, after which ministers were allowed to state their allegiances publicly.
Duncan Smith was one of a number of ministers, including Justice secretary Michael Gove, who left Downing Street after the cabinet meeting and went to Vote Leave's headquarters, where they endorsed the pro-Leave campaign group.
His comments run against those of the Prime Minister, who has argued that the UK is far strong and safer inside the EU.
Former deputy prime minister Lord Heseltine hit out at Duncan Smith's comments. "I hope that Duncan Smith's comments about terrorism are not typical of the scaremongering that could so easily characterise those arguing to leave Europe," he said.
Meanwhile, Jonathan Evans, former director general of MI5, also rubbished Duncan Smith's claims:
The UK's membership of the EU makes a positive contribution to our national security in a number of ways, for example through improving Police and judicial co-operation and by providing a multilateral framework for information and intelligence exchange with other countries.
Most importantly the EU underpins the overall stability of Europe, especially for newer entrants from the former Soviet bloc, in the face of external threats.
Open borders pose policing and intelligence challenges but are only one aspect of the overall security picture. In my experience 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.
Duncan Smith also said that reducing in-work benefits for migrants would not reduce immigration substantially as only a small number of people come to the UK for that reason alone.