Jacob Rees-Mogg has appeared to blame Chancellor Philip Hammond for "biased" Treasury forecasts which showed Britain would be worse off after Brexit whatever the outcome of negotiations.
Mr Rees-Mogg, who chairs the influential European Research Group of backbench Tory Leavers, stood by his claims that Treasury officials had been "fiddling the figures", but stressed "the blame must always be with ministers".
He also denied suggestions that he had demanded the resignation of Theresa May's Brexit "sherpa", Oliver Robbins, insisting "I have never attacked an individual named civil servant", because ministers are in charge of policy-making.
Speaking to postgraduate journalism diploma students at the Press Association in central London, the North Somerset MP said: "There are concerns that there are some people close to Government who are trying to undermine the Government's own policy."
He went on: "It's now been (made) clear we're not having the customs union, (it) is a reiteration of policy (that) the only person who seemed to be disagreeing with was the Chancellor of the Exchequer, and he ought to read up his constitution and think more carefully about what collective responsibility means."
He added: "When you take these models of what happens unless you stay in the customs union, they are all completely dependent on the inputs that you start with, and the inputs that they have started with are ones that lead to the conclusion that you have to stay in the customs union.
"Other economists have used different inputs and looked at different modelling of global trade which says we'll do extremely well by not being in the customs union.
"And so do I think civil servants are politically biased? Well I think the information the Treasury has produced is biased, but the blame must always be with ministers."
Mr Rees-Mogg also dismissed former Civil Service boss Lord O'Donnell's description of Brexiteers as "snake oil" salesmen who "don't like the idea of experts testing your products".
Lord O'Donnell had defended the Civil Service from Mr Rees-Mogg's attacks, insisting honesty and objectivity ran through the core of civil servants "like a stick of rock", and the forecasts would have been made in good faith.
He said: "My Lord O'Donnell has said that we're snake oil salesmen and he was cabinet secretary up until 2011 and in 2010 George Osborne, then chancellor, set up the Office for Budget Responsibility.
"Why did he set it up? He set it up because we needed an independent body because nobody trusted the figures coming from the Treasury, which were political.
"And O'Donnell was cabinet secretary when this was going on, was he resigning over that? Saying it was a great affront to the impartiality of the Civil Service to say that they'd fiddled the figures when Gordon Brown was chancellor? I think not, I think he stayed on happily in his job and therefore I think this is not unusual."