Influential Brexiteer Jacob Rees-Mogg | Oli Scarff/AFP via Getty Images Leading Brexiteer doubles down on claims civil service is ‘fiddling the figures’ Jacob Rees-Mogg says he stands by claims of anti-Brexit bias in UK Treasury.

Leading Brexiteer MP Jacob Rees-Mogg has refused to apologize for claiming the British civil service is attempting to influence Brexit policy.

Rees-Mogg, who chairs the Conservative Party's influential European Research Group, told the BBC on Saturday he stood by comments he made earlier this week, in the wake of a leaked government report that found that growth would be lower in three different Brexit outcomes than if the U.K. stayed in the European Union.

Rees-Mogg suggested officials working in the U.K. Treasury were seeking to keep the country in the EU customs union, and asked Brexit Minister Steve Baker on Thursday to confirm the allegation based on an off-the-record conversation with think tanker Charles Grant, who heads the Centre for European Reform.

Baker on Friday apologized to MPs for saying Rees-Mogg's account of the remarks was "essentially correct."

Speaking to BBC on Saturday, Rees-Mogg stood by his original claim, saying: "With the referendum and with the EU the Treasury has gone back to making forecasts. It was politically advantageous for them in the past. It is the same now ... So yes, I do think they are fiddling the figures."

Grant said he was "surprised" that Rees-Mogg had declined to apologize, saying it was normal for civil servants to speak to those working in think tanks. "That's how think tanks work," he said. "There's nothing wrong with that."

Mogg also said he fully backed Prime Minister Theresa May, describing her as "wonderfully stoic against all that's thrown at her."

CORRECTION: This article has been updated to clarify the nature of Jacob Rees-Mogg's accusations.