Lord Adonis has quit as the prime minister's infrastructure tsar, handing in his resignation over the government's handling of Brexit and accusing Theresa May of siding with Ukip and the "Tory hard right".
The Labour peer, a vocal critic of the decision to pull Britain out of the EU, said the government's approach to Brexit had made it impossible for him to continue as chairman of the National Infrastructure Commission.
In his letter to the prime minister, Lord Adonis claimed that Brexit is "causing a nervous breakdown across Whitehall and conduct unworthy of Her Majesty's Government".
And speaking to ITV News on Saturday, he described Nigel Farage as "essentially Prime Minister", and said Theresa May had put the country at risk of becoming increasingly akin to the National Front.
- Video report by ITV News political correspondent Carl Dinnen:
One Whitehall source said that Lord Adonis, whose frequent criticism of Brexit had caused outrage among Tory Eurosceptics, had simply "jumped before he was pushed" - a charge the peer said would be "outrageous" if true, as his position was intended to be independent.
In his resignation letter to Mrs May, Lord Adonis said that "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".
And he called Brexit "a dangerous populist and nationalist spasm worthy of [US President] Donald Trump".
Lord Adonis also said he felt "duty bound" to oppose the government's European Union (Withdrawal) Bill - which he called "the worst legislation of my lifetime" - when it comes to the House of Lords.
And he warned the government was "hurtling towards the EU's emergency exit with no credible plan for the future of British trade and European co-operation".
"If Brexit happens, taking us back into Europe will become the mission of our children's generation, who will marvel at your acts of destruction," he said.
Tory Eurosceptics welcomed Lord Adonis' departure, with the party's former leader Iain Duncan Smith calling it "long overdue".
Tory backbencher Nigel Evans dismissed Lord Adonis' objections to Brexit, saying: "As a democrat it is always good to have an unelected peer to tell the people how their referendum vote was wrong and he knows better than over 17.4 million voters."
But Lord Adonis has supporters across the political spectrum. Liberal Democrat leader Sir Vince Cable called him "one of the most thoughtful politicians around" and said his resignation was a "great shame".
- ITV News political correspondent Carl Dinnen on Lord Adonis' resignation:
A former Cabinet minister under Gordon Brown, Lord Adonis was appointed as chairman of the National Infrastructure Commission in October 2015.
Former chancellor George Osborne, who set up the National Infrastructure Commission (NIC) and appointed Lord Adonis, said he was "very sorry" the peer had resigned.
The London Evening Standard editor said: "He brought expertise and rare policy creativity, and will be a sad loss. I want to thank him personally for helping us establish our new NIC as a permanent improvement to long term thinking in the UK."
Lord Adonis also indicated he would have been compelled to quit over the government's handling of the East Coast rail franchise, accusing ministers of bailing out the firm running the service.
Transport Secretary Chris Grayling announced that a new East Coast Partnership will take on responsibility for both intercity trains and track operations on the route in 2020.
Virgin Trains East Coast, a partnership between Stagecoach and Sir Richard Branson's Virgin, had previously agreed to pay the government £3.3 billion to run the service until 2023.
Lord Adonis said it was an "indefensible decision" and "the bailout will cost taxpayers hundreds of millions of pounds, possibly billions".