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."

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