Former cabinet secretary Lord Gus O'Donnell said attacks on the civil service for allegedly sabotaging Brexit are "completely crazy", following claims officials are "fiddling the figures".
On Saturday, the leading Tory Brexiteer Jacob Rees-Mogg accused Treasury officials of "fiddling the figures" to show that Britain would be worse off outside the EU, whatever the outcome of the negotiations.
But in a staunch defence, 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: "I think that's completely crazy. 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.
The former cabinet secretary and head of the civil service compared their claims to the myth of the "stab-in-the-back" perpetuated by the German right following their country's defeat in the First World War.
His comments are likely to inflame the bitter war of words between some Brexiteers and elements in Whitehall who deeply resent the attacks on their impartiality.