Jo Johnson is pictured leaving 10 Downing Street in September.
London CNN  — 

Theresa May has lost another Johnson over Brexit.

Boris Johnson noisily quit as UK Foreign Secretary in July over the Prime Minister’s plans to leave the European Union. Now his lesser-known brother Jo, who was also a government minister, has followed him out.

The two Johnsons were on opposing sides in the 2016 referendum, but they have since come to the same conclusion: May’s plan to leave the EU is a dud.

“We are barrelling towards an incoherent Brexit that is going to leave us trapped in a subordinate relationship to the EU, with no say over the rules that will govern huge swathes of our economy,” Jo Johnson said in a video statement posted online. Such was his dissatisfaction with the deal being pursued by May that he had no option to resign his position as junior transport minister, he said.

Johnson said he intended to vote against May’s withdrawal agreement when it came before the UK parliament because it would represent “a terrible mistake.”

His resignation will likely come as a blow to May as she seeks to win the backing of her Cabinet in the coming days for the government’s proposed deal, which will then be put to a vote in parliament.

But the prospect of May being able to win the parliamentary vote took a knock Friday when Arlene Foster, leader of the Democratic Unionist Party (DUP), whose 10 MPs prop up the Prime Minister’s minority government, said she would also oppose the plan.

After nearly 18 months of grueling negotiations and with a March deadline looming, May is widely reported to be nearing a deal that would tie Britain to EU rules and regulations for a transition period. While May insists this would be time-limited, the exit clause is dependent on a final deal to avoid the need for infrastructure along the Irish border. That issue has proved so intractable that critics fear it could take years to resolve, leading supporters and opponents of Brexit to conclude it would represent the worst of both worlds.

May’s political calculation appears to be that MPs would vote for her deal over the alternative, crashing out of the EU without a deal. But Johnson said that was a false choice.

“To present the nation with a choice between two deeply unattractive outcomes, vassalage and chaos, is a failure of British statecraft on a scale unseen since the Suez crisis,” Johnson said.

Notably, Johnson said that the only way to resolve the political deadlock would be for the UK to hold another referendum, throwing his weight behind a growing campaign for a second public vote.

“Given that the reality of Brexit has turned out to be so far from what was once promised, the democratic thing to do is to give the public the final say,” he said.

May rejected calls for a second Brexit vote. “The referendum in 2016 was the biggest democratic exercise in this country’s history,” her spokesperson told CNN. “We will not under any circumstances have a second referendum.”

Boris Johnson tweeted his support for his brother’s move Friday, saying the pair may not have agreed about Brexit but were “united in dismay at the intellectually and politically indefensible” UK position.

“This is not taking back control. It is a surrender of control,” he said. “It does not remotely correspond to the mandate of the people in June 2016.”