Mr Duncan Smith – now a backbench MP – said he was becoming frustrated by the slow pace of talks and the EU’s reluctance to enter into trade discussions until it deems sufficient progress has been made on the question of the Irish border, and citizens’ rights.
He argued rather than adopt a piecemeal approach, EU negotiators should have agreed to hold “parallel talks” over the last few months.
In a column for the Telegraph, he wrote: “The reason we have not is down to one simple reason: it is that the EU has been from the beginning determined to control the process in order to extract the greatest payment from the UK, while stopping Britain gaining any advantage from our departure.”
Mr Duncan Smith pointed to Prime Minister Theresa May’s “generous offer” to uphold the rights of EU citizens, and continue to pay into the EU budget during the implementation phase which will follow UK’s exit from the EU in 2019.
However, the EU was demanding the jurisdiction of the European Court of Justice (ECJ) should be maintained, he said.
He added: “Notwithstanding the further £20 billion added last week, there were no further trade talks and yet further ramped demands from the Irish government on the Irish border and the EU on the Irish border and the EU on the ECJ.”
He also highlighted the recent botched attempt to solve the problem of the Irish border, and the apparent willingness to countenance a “regulatory alignment”, which proved unacceptable to DUP leader Arlene Foster, effectively scuppering the deal.
He said: “Ministers and officials are now telling everyone that they shouldn’t worry – it would be about regulatory alignment between the UK and the EU.
“Not the same as convergence, they tried to explain, but they are wrong.
“Ambiguity only feeds their [the EU’s] sense of control over these negotiations.
“For what becomes clearer by the day is that for the EU the real issue is whether they can succeed in stopping the UK from taking any competitive advantage once we leave.
“That is why the ambiguity will simply help them box us in in any future trade talks.”