Wycombe MP and Brexit minister Steve Baker has been accused of insulting civil servants after claiming their forecasts are always wrong.

Papers prepared by the Department for Exiting the EU and obtained by website Buzzfeed suggested that even with a comprehensive trade deal of the kind Theresa May is seeking, UK growth would be down by five per cent over the next 15 years.

This would rise to eight per cent if Britain left without a deal and was forced to fall back on World Trade Organisation rules.

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 on Tuesday that Civil Service forecasts are "always wrong".

He also accused Labour of adopting a Brexit strategy of "demoralisation, delay and revocation" as he attempted to play down the leaked analysis.

Mr Baker, referring to the Buzzfeed News website report on the leaked document, said: "The article is a selective interpretation of a preliminary analysis. It is an attempt to undermine our exit from the European Union."

Mr Baker said the Government is undertaking a wide range of analysis on Brexit, with the next stage, summarised in a draft paper brought together for ministers this month, "not yet anywhere near being approved by ministers".

He went on: "Even the ministerial team in my department has only just been consulted on this paper in recent days and we've made it clear it requires significant further work.

"In fact, I only saw this report myself this morning."

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.

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?"