Duncan Smith's comments come after documents revealed that a British diplomat suggested that visa-free travel for some Turkish nationals should be extended to the UK.
"These documents show that the government and the European Commission are perpetrating an enormous deceit on the British public - saying one thing in public, but quite another in private," Duncan Smith said.
"They say nothing is happening about Turkey, but this shows they are already making official plans to allow visa free access for Turkish citizens. The attempt to hide this, and to delay key decisions until after the referendum, is cynical in the extreme."
The document, revealed by the Sunday Times, revealed the diplomat, Janet Douglas, to have suggested Turks be considered "special passport" holders.
Douglas said that when the EU deal is implemented "we will need to develop our own lines on the UK's stance to visa-free travel for Turks.
"One option would be to assess again the possibility of visa travel for Turkish special passport holders which would be a risk, but a significant and symbolic gesture to Turkey."
Visa-free travel for Turks to the EU's Schengen Zone 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.
The foreign secretary and home secretary said: "The government's policy is, and will remain, to maintain current visa requirements for all Turkish nationals wishing to visit the UK, regardless of what arrangements other member states in the Schengen area may make with Turkey.
"Schengen visas do not give anyone the right to access the UK. The purpose of diplomatic telegrams is for our embassies around the world to feed back information on the position and views of foreign governments.
"They are reports from our diplomatic posts, not statements of British government policy."