FORMER cabinet secretary Lord Gus O’Donnell has dismissed accusations the civil service has tried to sabotage Brexit as a row over leaked assessments suggesting leaving the bloc will cause major damage to the economy continued.

On Saturday, hardline Tory Brexiteer Jacob Rees-Mogg accused Treasury officials of “fiddling the figures” to show the UK would be worse off outside the EU, whatever the outcome of the negotiations and even in the best scenario where it stayed in the single market.

But in a staunch defence of officials, Lord O’Donnell said 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.

Responding to claims officials distorted their analysis, the former civil servant told Peston On Sunday: “The truth is civil servants operate by the civil service code. The values are honesty, objectivity, integrity, impartiality.

“Their job is to look at the evidence and present it as best they can, analyse the uncertainties ... but that’s what they do, they’re objective and impartial.

“And I think what you find is that tends to get accepted very nicely when it agrees with someone’s prior beliefs, but actually, when someone doesn’t like the answer, quite often they decide to shoot the messenger.”

He was speaking to ITV’s Robert Peston, who said the civil service seemed to be facing “one of the most sustained attacks” on its integrity in living memory by serving ministers.

Lord O’Donnell added: “We look at the evidence and we go where it is.

“Of course if you are selling snake oil, you don’t like the idea of experts testing your products.

“And I think that’s what we’ve got, this backlash against evidence and experts is because they know where the experts will go.”

His remarks follow those of former Whitehall mandarin Lord Turnbull, who accused Brexiteers who blame civil servants for trying to sabotage Britain’s withdrawal from the EU of using tactics similar to those adopted by right-wing German nationalists in the 1930s.

As the Tory infighing continued over Brexit Home Secretary Amber Rudd said ministers will not be intimidated by threats of a leadership challenge to May from the Brexiteer wing of the party.

As the Cabinet’s Brexit sub-committee prepares to meet for two days of talks on the UK’s future relationship with the EU, Rudd insisted they would come forward with proposals which would command broad support in the party.

“I have a surprise for the Brexiteers, which is the committee that meets in order to help make these decisions is more united than they think,” she told BBC1’s The Andrew Marr Show. “We meet in the committee. We meet privately for discussions. I think that we will arrive at something which suits us all.” Rudd acknowledged the position would probably involve some form of customs “arrangement” or “partnership” with the EU, but said the Prime Minister had an “open mind” as to how that could be achieved. Her intervention came as the senior Tory, Eurosceptic, Bernard Jenkin launched a fresh attack on Chancellor Philip Hammond, accusing him of pursing his own policy on Brexit.

The Chancellor caused fury among Brexiteers when he suggested at the World Economic Forum in Davos that Britain’s relationship with the EU might only change “very modestly” after leaving.

Writing in The Sunday Telegraph, Bernard Jenkin said: “The PM can only command a majority in Parliament on her present policy.

“Nearly half David Cameron’s MPs voted Leave despite his patronage and pleadings. There would have been few Remain Tories if he had advocated Leave.

“Her MPs will back her, because we are overwhelmingly at one with the majority of the British people who now want a clean Brexit and an end to the present uncertainty.”