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Tory whip Gareth Johnson has quit the government to oppose Theresa May's Brexit deal in this week's meaningful vote.

The MP for Dartford, which voted leave in the 2016 referendum, wrote a letter to the prime minister saying the backstop in her agreement "gives our country no clear, unilateral path out of the European Union and ensures we will be fettered in our ability to negotiate trade deals".

"This agreement prevents us from taking back control and instead could leave us perpetually constrained by the European Union," he said. "Like you, I am not only a Conservative but I am also a committed unionist and I cannot accept the additional regulatory compliance required of Northern Ireland that would set it apart from the rest of the United Kingdom."

Stating the move is "possibly the hardest decision I have ever made", he added: "I have decided the time has come to place my loyalty to my country above my loyalty to the government."

Mr Johnson said at first he was "hopeful that changes could be made" to improve Mrs May's deal with the EU.

However, he said: "It is now clear to me that no significant change will be made to this agreement before the meaningful vote takes place.

"I hope, however, that even at this late stage, efforts will continue to change the terms of this agreement and the impact it would have on our country.

"We need to rediscover our confidence and belief in our country's ability to stand tall in the world without the European Union overseeing and managing our future."

Key Brexit moments of 2018: in pictures 23 show all Key Brexit moments of 2018: in pictures 1/23 Prime Minister Theresa May speaks during a Cabinet meeting at Chequers in July PA 2/23 The Cabinet thrashed out a Brexit blue print PA 3/23 The away day meeting PA 4/23 Boris Johnson leaves Carlton House Terrace in Westminster after he resigned as Foreign Secretary PA 5/23 David Davis pictured after quitting the government, resigning his post as Brexit Secretary AFP/Getty Images 6/23 New role: Jeremy Hunt was appointed as Foreign Secretary in July Getty Images 7/23 Dominic Raab took over the role of Brexit Secretary in July AP/Matt Dunham 8/23 Exotic spresm: Liberal Democrats Leader Sir Vince Cable following his speech at the party conference PA 9/23 More than half a million protesters descended on London for the People's Vote March in October Sky News 10/23 Demonstrators called for a People's Vote on Brexit during a march in Liverpool earlier this year Getty Images 11/23 Chancellor of the Exchequer, Philip Hammond, presented the red Budget Box as he revealed the government's financial plans in October Getty Images 12/23 It was the last Budget before Brexit AFP/Getty Images 13/23 Mr McDonnell and Jeremy Corbyn listen as Chancellor Philip Hammond delivers his Budget statement AFP/Getty Images 14/23 New Brexit Secretary Stephen Barclay after Dominic Raab quit the post in November Getty Images 15/23 Esther McVey also resigned as Work and Pensions Secretary 16/23 Sir Keir Starmer opened the debate on a motion to hold the Government in contempt of Parliament on December 4 AFP/Getty Images 17/23 Sir Graham Brady (centre), chairman of the 1922 Committee, announces that Theresa May has survived a confidence vote on December 12 PA 18/23 Theresa May in Downing Street after winning the confidence vote Jeremy Selwyn 19/23 The Prime Minister confronts Jean-Claude Juncker at an EU summit in December 20/23 'You called me nebulous' Philip Sime/Twitter 21/23 Jeremy Corbyn was accused of mouthing the words "stupid woman" during the last PMQs of the year 22/23 Jeremy Corbyn responds in the Commons amid the row PA 23/23 Jeremy Corbyn denied reports he muttered the words "stupid woman" AFP/Getty Images 1/23 Prime Minister Theresa May speaks during a Cabinet meeting at Chequers in July PA 2/23 The Cabinet thrashed out a Brexit blue print PA 3/23 The away day meeting PA 4/23 Boris Johnson leaves Carlton House Terrace in Westminster after he resigned as Foreign Secretary PA 5/23 David Davis pictured after quitting the government, resigning his post as Brexit Secretary AFP/Getty Images 6/23 New role: Jeremy Hunt was appointed as Foreign Secretary in July Getty Images 7/23 Dominic Raab took over the role of Brexit Secretary in July AP/Matt Dunham 8/23 Exotic spresm: Liberal Democrats Leader Sir Vince Cable following his speech at the party conference PA 9/23 More than half a million protesters descended on London for the People's Vote March in October Sky News 10/23 Demonstrators called for a People's Vote on Brexit during a march in Liverpool earlier this year Getty Images 11/23 Chancellor of the Exchequer, Philip Hammond, presented the red Budget Box as he revealed the government's financial plans in October Getty Images 12/23 It was the last Budget before Brexit AFP/Getty Images 13/23 Mr McDonnell and Jeremy Corbyn listen as Chancellor Philip Hammond delivers his Budget statement AFP/Getty Images 14/23 New Brexit Secretary Stephen Barclay after Dominic Raab quit the post in November Getty Images 15/23 Esther McVey also resigned as Work and Pensions Secretary 16/23 Sir Keir Starmer opened the debate on a motion to hold the Government in contempt of Parliament on December 4 AFP/Getty Images 17/23 Sir Graham Brady (centre), chairman of the 1922 Committee, announces that Theresa May has survived a confidence vote on December 12 PA 18/23 Theresa May in Downing Street after winning the confidence vote Jeremy Selwyn 19/23 The Prime Minister confronts Jean-Claude Juncker at an EU summit in December 20/23 'You called me nebulous' Philip Sime/Twitter 21/23 Jeremy Corbyn was accused of mouthing the words "stupid woman" during the last PMQs of the year 22/23 Jeremy Corbyn responds in the Commons amid the row PA 23/23 Jeremy Corbyn denied reports he muttered the words "stupid woman" AFP/Getty Images

Just over 65 per cent of Mr Johnson's Dartford constituency voted leave in the 2016 Brexit referendum.

His letter added: "Along with nearly two thirds of my constituents and a majority of the country, I supported 'leave' in the referendum as I wanted the UK to take back the sovereignty we had lost during our membership of the European Union.

"Unfortunately, this agreement prevents us taking back control and instead could leave us perpetually constrained by the European Union."

Mr Johnson's announcement comes as Mrs May received a joint letter from European Council president Donald Tusk and European Commission president Jean-Claude Juncker which said they were "not in a position" to rewrite or amend the Withdrawal Agreement secured last year.

But they assured the Prime Minister that the EU "does not wish to see the backstop enter into force", as it would represent a "sub-optimal trading arrangement for both sides".

The EU presidents stated: "Were the backstop to enter into force in whole or in part, it is intended to apply only temporarily, unless and until it is superseded by a subsequent agreement."