Senior government adviser Andrew Adonis has stirred controversy by comparing Brexit to appeasing Adolf Hitler in the 1930s and warning that Britain is in “serious danger”.

“My language is usually pretty subdued in politics,” said Adonis, a member of the House of Lords who was appointed chairman of the National Infrastructure Commission by arch-europhile George Osborne and reappointed by his successor Philip Hammond, in an interview with The House magazine.

“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,” he said.

“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.”

“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.”

Yes, I did say this and I mean it. At v least, we must stay in Single Market & Customs Union, the EU's key economic institutions #besensible https://t.co/0fVfl6QZoG — Andrew Adonis (@Andrew_Adonis) July 13, 2017

The comparison prompted an angry response from Brexit campaigners such as former Tory leader Iain Duncan Smith, who said he was “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,” he added.

Tory backbencher Peter Bone said Adonis should be fired from his Government position, and colleague Andrew Bridgen agreed.

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

A spokesman for the Prime Minister said she disagreed with Lord Adonis’s views but would not fire him.

Adonis himself was unrepentant: “I did say this and I mean it,” he declared on Twitter.

“At [very] least, we must stay in Single Market & Customs Union, the EU’s key economic institutions,” he asserted.

Adonis favours a form of so-called ‘Soft Brexit’ which would see Britain continuing to pay into the EU budget, accepting EU directives and regulations, remaining subject to the European Court of Justice, and staying inside its Customs Union and the Single Market.

The Labour peer claims this would be “a fair deal between Britain and our European partners to mutual advantage”.