THERESA May’s “infrastructure czar” has quit, saying Brexit has caused a “nervous breakdown across Whitehall” and “conduct unworthy” of the government.

Lord Adonis, who served as the chairman of the National Infrastructure Commission from April, said the Prime Minister had allied herself “with Ukip and the Tory hard right”.

Adonis, who was a former transport minister in the last Labour government, said he felt duty bound to oppose the European Union (Withdrawal) Bill when it comes to the House of Lords next year. and therefore couldn’t remain in post.

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In his resignation letter, Adonis said: “Brexit is a dangerous populist and nationalist spasm worthy of Donald Trump. After the narrow referendum vote, a form of associate membership of the EU might have been attempted without rupturing Britain’s key trading and political alliances. Instead, by allying with UKIP and the Tory hard right to wrench Britain out of the key economic and political institutions of modern Europe, you are pursuing a course fraught with danger.”

He added: “Brexit is causing a nervous breakdown across Whitehall and conduct unworthy of Her Majesty’s Government.”

Adonis went on to say he felt he would have been forced to quit regardless of Brexit after the government’s “bailout” of the Stagecoach and Virgin East Coast franchise, suggesting it will cost the taxpayer “hundreds of millions of pounds”, which he called “more inexcusable given the Brexit squeeze on public spending”.

Adonis’s position was thought to be on a shoogly peg. He held on to his his role in April, and avoided being sacked in July, even though he compared Brexit to “Nazi appeasement”. A government source said he had “walked through the door before he was pushed”.

Iain Duncan Smith MP told Sky News: “His departure is long overdue. These comments come from a man who never got elected – doesn’t he think it a touch elitist to have been placed in the House of Lords without ever having fought an election to then pontificate on what he calls populism but which many of us would call democracy.”

Jacob Rees Mogg MP told Sky News: “It is characteristic of the Blairite dismissiveness of the electorate. Not dangerous, populist and nationalist but arrogant, out of touch and elitist.”

Responding, Liberal Democrat leader Vince Cable said: “Lord Adonis is one of the most thoughtful politicians around. This is why he has so many friends and political admirers beyond the Labour Party.

“It is, then, a great shame that he is no longer leading Britain’s infrastructure programme. Yet he felt there was no other option but to resign”.