The former Conservative leader Iain Duncan Smith has accused the government of “perpetrating an enormous deceit” on the British public, as it emerged the government is secretly planning to grant visa-free travel to up to 1.5 million Turks.

Documents leaked to The Sunday Times yesterday revealed that British diplomats have suggested to ministers that they grant access to the UK for 1.5 million Turkish “special passport holders” as a significant and symbolic gesture to Turkey.”

They also showed that EU diplomats were trying to keep the deal with Turkey on visa-free access to the Schengen area under wraps until after the referendum next week.

Downing Street confirmed that the documents were genuine, but denied that it was considering granting visa-free access to some Turks.

But former work and pensions minister Iain Duncan-Smith accused the Prime Minister David Cameron of being “in cahoots” with the Commission to perpetrate “an appalling deceit” on the British public.

“While the prime minister and the government have been denying any kind of deals with Turkey, this is absolute proof that behind the scenes they are already planning to bring the Turks in.

“The government is clearly working with the EU to keep a lid on it until after the referendum when all the decisions will surface too late.”

Telegrams within the leaked cache also reveal that home secretary Theresa May has been warned by Europol that uncontrolled migration does pose a terror threat to Britain, something she has repeatedly denied.

A memo from the UK mission in Brussels read “Europol . . . confirmed there was some emerging analysis linking migration with terrorism — for example, the false Syrian passports provided to the Paris bombers came from the same source as used by other smuggling networks.”

The memo flies in the face of the official line from the Remain campaign, of which the home secretary is part, which insists that remaining within the EU is the safer option in terms of the terrorism threat.

In a speech earlier this year Ms May insisted that Britain was safer from terrorism inside the EU because of the European Arrest Warrant, an extradition agreement between EU states, and because of the Passenger Name Records Directive which requires airlines to collect and store detailed information on each and every passenger passing through European airports, including details of which meals they have requested on the flight.

She also said it was “not true” that having no control over our borders made us less safe.

Meanwhile the minister for employment, Priti Patel, has highlighted research by Migration Watch showing that Turkish accession to the EU could result in 100,000 Turks arriving in the UK every year.

The figure was “enough, on its own, to break David Cameron’s manifesto pledge to reduce migration to the tens of thousands,” she said.

Migration Watch chairman Lord Green of Deddington said: “This report is a final wake up call.

“If we remain in the EU there will be nothing to stop a continuing rapid increase in our population. This would change our country for ever against the express wishes of a very large majority of our fellow citizens.”

David Cameron hit back, in turn accusing the Leave campaign of lying over Turkey.

“There is no prospect of Turkey joining the EU within decades,” he told the BBC’s Andrew Marr. “It’s simply not an issue, it’s a total red herring and the out campaign know that.

“Some of them have admitted to it so they should stop frightening people because it’s not going to happen.”

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