In an excoriating takedown of Ms May’s approach set out by the cabinet at Chequers, the former foreign secretary told the House of Commons it is not too late for Britain to change its course.
Mr Johnson, who quit his government job over the Chequers deal, urged Ms May to stick to the vision for Brexit she set out in her Lancaster House speech 18 months ago, but argued she had since undertaken a “stealthy retreat”.
His criticism comes at an unstable time for Ms May’s leadership, with the prime minister having only narrowly avoided a commons defeat on Tuesday that could have triggered a ‘no confidence’ vote, and Tory backbenchers plotting to overthrow her.
Mr Johnson delivered the attack during his resignation speech, surrounded by supportive Brexiteer Tory MPs, at the opposite end of the government benches from the ministerial despatch box he has spoken from over the past two years.
The former minister, who himself harbours ambitions to be prime minister, said: “Let us again aim explicitly for that glorious vision of Lancaster House – a strong, independent self-governing Britain that is genuinely open to the world.
“Not the miserable permanent limbo of Chequers, not the democratic disaster of ongoing harmonisation with no way out and no say for the UK.”
“We need to take one decision now before all others, and that is to believe in this country and what it can do.”
In the wake of the speech, Mr Johnson’s odds of becoming the next prime minister were pitched at 5/1 by bookmaker William Hill, behind only Labour leader Jeremy Corbyn.
Knowing that his own leadership ambitions would be in the spotlight, the Uxbridge and South Ruislip MP sought to focus his criticism on the prime minister’s policy and started by praising her “courage and her resilience”.
But that would only go a little way to balance the condemnation of her approach, as he lamented what he claimed was the prime minister’s abandonment of the vision she set out in her Lancaster House speech.
“In the 18 months that have followed it is as though a fog of self-doubt has descended, and even though our friends and partners liked the Lancaster House vision – it was what they were expecting from an ambitious partner, what they understood, even the commentators liked it, and the markets liked it… we never actually turned that vision into a negotiating position in Brussels, and we never made it into a negotiating offer.
“Instead we dithered and we burnt through our negotiating capital. We agreed to hand over a £40bn exit fee, with no discussion of our future economic relationship.
“We accepted the jurisdiction of the European Court over key aspects of the withdrawal agreement, and worst of all – we allowed the question of the Northern Irish border, which had hitherto been assumed on all sides to be readily soluble, to become so politically charged as to dominate the debate.”
Mr Johnson claimed it is possible to avoid a hard border in Northern Ireland while also having different regulation on the north and south side of the border – something denied by Brussels – and having no border checks, facilitated by technical solutions.
He said solutions that he and ex-Brexit secretary David Davis had put forward were never examined by Ms May’s officials, as if they had become “intellectually undesirable”.
Mr Johnson went on to directly compare aspects of the Lancaster House speech to the deal agreed to by the cabinet at Chequers, pointing out how the latter fell short on each count.
In direct violation of the Lancaster House speech, Mr Johnson argued that the Chequers deal would see the UK become “rule-takers”, would see it holding on “to bits of membership” and under the jurisdiction of the European Court.
He said: “We are volunteering for economic vassalage, not just in goods and agrifoods, but we will be forced to match EU arrangements on the environment and social affairs and much else aside.”
In an appeal to his Conservative colleagues, the ex-minister said: “It is not too late to save Brexit. We have time in these negotiations.
“We have changed tack once and we can change again. The problem is not that we failed to make the case for a free trade agreement of the kind that we spelt out at Lancaster House.
“We haven’t even tried. We must try now because we will not get another chance to get it right.”
In a message directed at his former cabinet colleague and one-time leadership rival Michael Gove, he said that it is “nonsense” that the UK can make a “botched treaty” now then simply get out of the EU and try to change it later.
He ended by saying that the government had to acknowledge that the approach it was taking would hamper the UK’s ability to make free trade deals, because to do anything else would be making “the fatal mistake of underestimating the intelligence of the public”.
Ms May was giving evidence to a select committee while Mr Johnson was delivering his speech in the Commons. Asked if she would watch it on catch-up, she said: “I think I’ll probably be doing my red box.”
Meanwhile, anti-Brexit Labour MPs attacked Mr Johnson’s speech, including David Lammy who branded it “dreary” and ”self-important”.
Wes Streeting, Labour MP and supporter of the People’s Vote campaign for a second referendum, said: “Boris Johnson’s speech was a total damp squib.
“All it shows is that while the car is careering towards the Brexit cliff edge, senior Conservatives are still fighting over what song to play on the stereo.”
Brexiteers have said that they have enough names – 48 are required in total – to force a vote of no confidence in the prime minister, but have refrained from handing them in up to now.
They have already managed to force changes in Ms May’s plans, consisting of four amendments, one of which would act to keep the UK out of a customs union, and which some have claimed effectively leaves her Chequers deal “dead in the water”.
At a meeting of the backbench 1922 Committee on Wednesday, one MP who had previously written a letter calling for a vote of no confidence withdrew it and said the PM should now be given a chance to negotiate with the EU.
The European Union is yet to say whether the plan is “workable” in their eyes, though some officials have said off the record that the plan makes the “same mistake” the UK has made since the start of the negotiations, in that it is seeking access to the single market without adherence to the EU’s four freedoms – including the free movement of people.