Rees-Mogg, who leads the pro-Brexit European Research Group of Conservative MPs, refused to apologise for accusing civil servants of trying to undermine the Brexit process.
He said the Treasury, before and since the EU referendum in June 2016, had found it “politically advantageous” to tilt their forecasts.
“I do think they’re fiddling the figures,” Rees-Mogg said, speaking to the BBC.
A Treasury spokesperson said: “Both Treasury ministers and officials are working hard to deliver the best Brexit deal for Britain. The Prime Minister and the chancellor have said repeatedly that we will be leaving both the single market and the customs union. Any suggestion to the contrary is simply false”.
The row comes in the aftermath of leaked Department for Exiting the EU forecasts which found that leaving the EU customs union would be significantly worse for the economy than remaining in it, in line with the vast majority of economists’ predictions.
The government’s official policy is to leave the customs union, which would enable the UK to strike trade deals with other nations. However, it would likely also entail the imposition of border checks on goods flowing between the UK and the EU, a major change from the current frictionless system.
The row within the Conservative party has drawn in government figures, with junior Brexit minister Steve Baker yesterday apologising after not correcting a question from Rees-Mogg in Parliament, in which the backbencher said he believed the Treasury was trying to influence the debate.
Rees-Mogg accused government economists of deliberately developing their forecasting model to prove that staying in the Single Market or the customs union would be better for the economy than leaving.
Today Rees-Mogg refused to back down from his accusations, for which he has not provided evidence, saying the situation was now “much more serious than before”. The Treasury seems “to be running policy that is not government policy” and then briefing journalists, Rees-Mogg said.
"If you look at the forecasts the Treasury made before the referendum, they were a humiliation. They were clearly politically influenced," he said.