Justice secretary Michael Gove has made the security argument for Brexit, pointing to Turkey's hypothetical accession to the EU as a reason to vote for the UK to leave the EU.
Speaking at a Vote Leave event, Gove said it was both British and EU policy for Turkey to become a member of the EU.
However, Prime Minister David Cameron has previously said Turkey would not join the union for the foreseeable future, adding that each country had a veto. Turkey, he said, has to reach minimum standards to make it eligible - and at this rate it won't join "until the year 3000".
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Still, Gove said Turkey had threatened to end co-operation in stopping migration unless a deal with the EU included visa-free travel, the BBC reported.
The visa-waiver programme is part of a wider deal agreed earlier this year, in which Turkey agreed to take back migrants entering the EU in exchange for sweeteners.
Gove said: "With the terrorism threat only growing, it's hard to see how it could possibly be in our security interests to open visa free travel to 77m Turkish citizens and create a border free zone from Iraq, Iran and Syria to the English Channel.
"It's hard to see how such a course is wise when extremists everywhere will believe the West is opening its borders to appease an Islamist government."
His key ally Boris Johnson has also said: "I think the chances of the Turks readily acceding to the European Union are between, you know, nil and 20 per cent… Well, probably lower than that. I mean, it's not going to happen in the… foreseeable future.
"They think the idea of suddenly 75m Turks having the, you know, and all of those coming in to Turkey notionally having rights of free travel, visa-free travel to the EU, that's just simply not on the cards."
The move to security comes as the economic and immigration arguments slog on. However, recent polling data suggests that Remain has won the economic argument and Leave has won the immigration argument.