Minister accused of failing to challenge 'half-baked' Brexit 'conspiracy theory'

Jacob Rees-Mogg asked Mr Baker to confirm if he had been told by the Centre for European Reform's Charles Grant that "officials in the Treasury have deliberately developed a model to show that all options other than staying in the customs union were bad and that officials intended to use this to influence policy".

Mr Baker said the account was "essentially correct", adding: "At the time I considered it implausible because my direct experience is that civil servants are extraordinarily careful to uphold the impartiality of the Civil Service."

Mr Grant himself said he had told Mr Baker at the Conservative Party conference in October that he was aware of research from the Treasury which apparently showed the costs of leaving the customs union outweighed the economic benefits of the UK being able to sign free trade deals around the world.

And Mr Grant distanced himself from claims the Treasury is deliberately trying to influence Brexit policy.

"I did not say or imply that the Treasury had deliberately developed a model to show that all non-customs union options were bad, with the intention to influence policy," he said.