Britain is headed for the status of "colony" to the European Union, Boris Johnson has warned, just hours after news broke of his resignation as foreign secretary.

"We are truly headed for the status of a colony - and many will struggle to see the economic or political advantage of that particular arrangement," Johnson said in a resignation statement.

Earlier on Monday, Johnson became the third government official to walk away from the government over Prime Minister Theresa May's soft Brexit plan which has angered many Conservatives.

Johnson also stated in the letter that, by "surrendering control over our rulebook," the UK will make it "much more difficult to do free trade deals." What is most disturbing, he said, is that the unacceptable plan laid out by May is the country's "opening bid."

I am proud to have served as Foreign Secretary. It is with sadness that I step down: here is my letter explaining why. pic.twitter.com/NZXzUZCjdF — Boris Johnson (@BorisJohnson) July 9, 2018

"This is already how we see the end state for the UK - before the other side has made its counter-offer," he wrote. "It is as though we are sending our vanguard into battle with the white flags fluttering above them."

Johnson's resignation is yet another sign of chaos within May's government, after her EU exit plan angered staunch Brexiteers who say the deal isn't good enough and concedes too much to the bloc.

The opposition is equally as unhappy, with Labour leader Jeremy Corbyn asking on Monday: “How can anyone have faith in the prime minister getting a good deal with 27 EU governments when she can’t even broker a deal in her own cabinet?”

Some have called for May to step down, while reports are suggesting a possible no-confidence vote. When asked about a possible challenge to her leadership on Monday, she simply replied: "Nice try, but I'm getting on with the job of delivering what the British people want," Reuters reported.

In a letter, May told Johnson she was a "little surprised" to receive his resignation letter following productive Brexit talks on Friday.

"I am sorry - and a little surprised - to receive it after the productive discussions we had at Chequers on Friday, and the comprehensive and detailed proposal which we agreed as a Cabinet," May wrote, adding that the proposal "will honor the result of the referendum and the commitments we made in our general election manifesto to leave the single market and the customs union."

“A little surprised” - Theresa May to Boris Johnson pic.twitter.com/1zUpFfbHX9 — Sam Coates Times (@SamCoatesTimes) July 9, 2018

May also wrote that she had allowed Cabinet members "considerable latitude to express their individual views," but that such time has passed. "If you are not able to provide the support we need to secure this deal int eh interests of the United Kingdom, it is right that you should step down," she wrote.

Johnson served as foreign secretary for almost exactly two years, taking up the post on July 13, 2016.

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