Brexit minister Steve Baker has been accused of insulting civil servants after claiming their forecasts are always wrong.
The FDA, which represents senior public servants, said it was extraordinary that a minister should tell the Commons he would not believe analysis carried out in Whitehall.
Quizzed by MPs about leaked assessments on the impact of Brexit reported by Buzzfeed News, Mr Baker said it is "not a forecast for our preferred outcome of the negotiations".
Mr Baker told the Commons that Civil Service forecasts are "always wrong".
FDA general secretary Dave Penman accused Mr Baker of having a "fingers-in-ears" approach to Brexit.
He said: "Steve Baker's comments in Parliament are supposed to represent the considered view of the Government.
"His remarks today not only insult the dedicated professionals working in his department and across the Civil Service, but they epitomise the current state of affairs in government.
"We have just witnessed the extraordinary scene of a serving minister telling the House that, whatever analysis his own department comes up with, he simply won't believe it.
"The public will rightly ask how this fingers-in-ears approach is supposed to help the country navigate such a complex and unprecedented challenge as leaving the EU.
"How can civil servants in Mr Baker's department, who are working harder than ever before, now have confidence in a minister who stands at the despatch box and openly questions their professionalism?
"The real question, however, is how can a minister prepared to undermine the government he serves retain the confidence of the Prime Minister?"