Former British foreign secretary Boris Johnson on Tuesday excoriated Prime Minister Theresa May’s road map to exiting the European Union as a sad, weak and dangerous mistake.

In a brutal speech at the annual Conservative Party conference, Johnson blasted his fellow Tory, saying her approach is not only “politically humiliating” but “a cheat” against everyone who voted for Brexit.

The loquacious Etonian famous for his blond bird’s nest of hair almost ran out of words to condemn May’s strategy for the divorce deal of the decade.

“This is not pragmatic, it is not a compromise,” Johnson warned. “It is dangerous and unstable — politically and economically.”

“My fellow Conservatives, this is not democracy. This is not what we voted for. This is an outrage. This is not taking back control. This is forfeiting control.”

The hall was overflowing. Delegates queued for more than an hour to get in to hear the speech, which dominated British airwaves despite being presented as a “fringe event” at the conference, upstaging humdrum addresses by May’s cabinet ministers, who talked taxes, housing and the perils of food waste in a mostly empty hall next door.

Prime Minister Theresa May’s Brexit plan is “dangerous and unstable — politically and economically,” former British foreign secretary Boris Johnson said during the Conservative Party annual conference in Birmingham, England, on Oct. 2, 2018. (Rui Vieira/AP)

Michael Deacon, a columnist for the Telegraph wrote that Johnson “humiliated” May at her own party — “and Tory members cheered every word.”

Conservatives have long suspected that what Johnson wants most of all is to become prime minister some day, but they are doubtful he will mount a challenge right now. “The voters would punish us for staging a leadership fight in the middle of Brexit,” said Tom Lees, an engineer and Conservative party member attending the conference.

The flamboyant Johnson was the face of the Brexit campaign in the summer of 2016, when citizens voted 48 to 52 percent to split from the E.U.

Back then, Johnson and his allies promised Brits that leaving the world’s biggest free-trade zone would be easy, allowing London to “take back control” from faceless bureaucrats in Brussels, while slashing the number of immigrants and providing a windfall of cash to save Britain’s financially strapped universal health-care service.

But Brexit has been nothing if not chaotic and costly — as the pound sterling has been hammered and economists say uncertainty over future relations with Europe has slowed growth.

Hardcore Brexiteers like Johnson hate May’s plan. They deride it as “Brexit in name only,” a capitulation to wily continentals. They want a clean, complete break from the Europe’s rule-making, so a “global Britain” could flex its new regulatory freedoms to strike trade deals on its own.

Johnson resigned in July as the foreign secretary after May persuaded her fractured cabinet — including Johnson — to approve her “Chequers plan,” named after her official countryside estate.

May’s plan seeks to protect as much as possible frictionless, tariff-free trade of goods with the bloc by agreeing to follow a “common rule book” with E.U. regulations for safety and standards for goods, while Britain goes its own way on financial services.

May has struggled to come up with a plan that avoids a hard border — for passport control, customs duties and inspections — between Northern Ireland, which remains a part of the U.K., and the Republic of Ireland, which remains in the E.U.

To add to May’s challenges, the Democratic Unionist Party of Northern Ireland — which is propping up the Conservative’s minority government — warned they would crash the ruling coalition if May agreed to erect a border in the Irish Sea.

May’s 100-page Chequers plan is not only unpopular at home and among her own party members, but also in Brussels. At a recent meeting of European leaders, Donald Tusk, the European Council president, warned May that her plan “will not work” and asked her to do better.

British Environment Minister Michael Gove, a top Brexiteer, suggested the government should back May, adding any deal struck now could be renegotiated later.

Johnson told his audience that was a “total fantasy.” He warned, “The opposite will happen.”

He called the half-in, half-out approach a recipe for continued acrimony.

Johnson said May’s plan to harmonize rules and regulations with Europe is not what he campaigned for — or what the people want.

Hard-line Brexiteers have been pushing the government to “chuck Chequers” and try for a trade deal similar to the one negotiated between Canada and the E.U. But that deal took years to hammer out and requires border controls and inspections.

Chancellor of the Exchequer Philip Hammond, who backs May’s compromise, called Johnson “a big-picture man” who is weak on the details.

“This is a very detailed and complex negotiation. It doesn’t require big, sweeping statements,” Hammond told the BBC.

So far, though, May’s negotiators have not managed to make a deal with the E.U., either.

Britain is willing to listen to “alternative ways” of leaving the union, said the new Brexit secretary, Dominic Raab, who complained at the Tory conference that the Europeans needed to “get serious.”

European leaders also have grumbled that Britain bickers, delays and dawdles — and that it wants to have its cake and eat it, too, as Johnson once famously boasted.

It did not help matters that Britain’s new foreign secretary, Jeremy Hunt, compared the E.U. to the Soviet Union.

“The E.U. was set up to protect freedom. It was the Soviet Union that stopped people leaving,” Hunt said Monday. “If you turn the E.U. club into a prison, the desire to get out won’t diminish, it will grow, and we won’t be the only prisoner that will want to escape.”

After Johnson’s speech, which was repeatedly interrupted by applause, Graham Jones, a Tory member and West Lancashire lawyer, said, “We hope that Theresa May hears Boris and gets the message.”

May told the BBC that Johnson put on “a good show,” but she was “cross” with him for being reckless about Northern Ireland’s concerns.