A senior Government adviser is facing calls for his sacking after comparing a so-called “hard” Brexit to British appeasement of the Nazis in the 1930s.

Labour peer Lord Adonis, who chairs the Government’s National Infrastructure Commission, also compared leaving the European Union’s single market and customs union to the end of the British Empire.

He said the UK is in “serious danger” of getting Brexit wrong and warned the country faces a steep decline in living standards compared with France and Germany unless the country can “have our cake and eat it”.

Lord Adonis told the House magazine: “My language is usually pretty subdued in politics.

“But anyone with a historical sense – and I’m a historian – recognises that leaving the economic institutions of the European Union, which have guided our destiny as a trading nation for half a century, is a very big step and the importance can’t be overemphasised.

“To my mind, it’s as big a step that we’re taking as a country as decolonisation in the 1950s and 60s and appeasement in the 1930s.

Lord Adonis

Lord Adonis

“We got it right on decolonisation; we got it wrong on appeasement and I think we’re in serious danger of getting it wrong in the way that we leave the EU.

“If we can’t have our cake and eat it then we face a serious relative decline in our living standards compared with France and Germany and I don’t believe the British people will put up with that.

“So we would, in that event, I believe face a crisis.

“It may be a crisis played out over quite a number of years – which, after all, is what happened with appeasement – but there will be a crisis.

“It’s important for political leaders like me to sound the alarm bell because it’s important to understand what might be at stake: in 18 months’ time, people may require visas to go to France.”

His comments sparked calls for his sacking from Brexit-backing Tory MPs.

Former Tory leader Iain Duncan Smith told the Sun: “I am astonished and appalled that someone who considers himself to be intelligent should have selected such a comparison given all the appalling violence and death that Hitler visited on Europe and the rest of the world.

“I find his comments deeply offensive, as I believe most of the British people will too.”

Tory MP Peter Bone told the newspaper: “He should be fired.”

Fellow Tory backbencher Andrew Bridgen said: “Of course he should be removed from any advisory role to government. He doesn’t really believe in democracy – just like Hitler.”

Later, Lord Adonis maintained his position and urged Labour to back staying in the single market and customs union.

Asked if he was sticking by the comments, and where that leaves the party, the peer told BBC Radio 4’s Today programme: “Leaving the European Union is clearly going to have very, very serious economic consequences.

“The point I’m making is that it’s the worst economic mistake we will have made in this generation if we do leave and do a hard Brexit.

“My view is that the Labour Party should stand behind staying in the single market and the customs union – I’ve argued that in the House of Lords, I couldn’t have been clearer.”

Lord Adonis said he believed voters back his stance and that it was a “matter of time” before Labour came “in line with the country”.

“There is no way that the Labour Party, as the party representing the working people of this country, is going to take a position that sacrifices their jobs and makes them poorer, and if it were to take that position, and we were to do a hard Brexit, then I do believe this would be the worst mistake this country has made since appeasement in the 1930s and it will impoverish millions of working people,” he said.

Labour’s divisions over Brexit erupted into the open last month after 50 MPs defied leader Jeremy Corbyn to back a Commons motion calling for Britain to remain in the single market.

Mr Corbyn, who is committed to leaving the trade bloc to end free movement of EU citizens, responded by sacking three shadow ministers.