The former Conservative Party leader even claimed there was an empty chair — known as the 'German chair' — in Downing Street, which featured as a symbol of influence Berlin held in the negotiations.
"It's like they were sitting in a room, even when they were not there," he told British tabloid, the Sun.
"There was a spare chair for them — called the German Chair. They have had a de facto veto over everything."
Duncan Smith said Cameron dropped demands for greater border control from a November 2014 speech; an incident that he believes highlighted the German influence over British negotiations.
"I saw the draft… I know that right up until the midnight hour, there was a strong line in there about restricting the flow of migrants from the European Union — an emergency break on overall migration," he said.
That was dropped, literally the night before. And it was dropped because the Germans said ‘if that is in the speech, we will have to attack it.' The whole thing was shown to them.
"The Germans said from the outset, you are not getting border control. Full stop."
While fellow Euroskeptics used the claims to push for Britain to leave the EU, others accused Duncan Smith of blatantly lying.
"This, I think, is just not true," German MEP and senior negotiator during the talks, Elmar Brok, told the BBC.
"Britain got what is possible under these circumstances and that was a great success story.
"It is not easy to sell in countries like Poland and others that Britain got this special right and therefore Duncan Smith is trying to explain to its people that Germany has a direct impact on British policy, that is just wrong. Mr Duncan Smith is lying to his own people."