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Justine Greening vowed to quit the Tory party at the next election, as she launched a blistering attack on Boris Johnson and his election threat.

The ex-international development secretary said her concerns that the Tory Party was morphing into Nigel Farage's Brexit Party had "come to pass", and said that the Prime Minister was offering the country a "lose-lose" situation by threatening a general election.

The MP for Putney, Roehampton & Southfields, a Remain campaigner, also told Radio 4's Today show that she feels she can make a bigger difference outside of Parliament.

She later shared her letter of resignation on Twitter, saying she had been "proud" to be "the first openly LGBT female Cabinet Minister" but that she was "deeply concerned" by the party's Brexit approach.

Mr Johnson is expected to request a general election on October 14 if MPs back the cross-party move to seize control of Commons business when Parliament returns from recess today.

The former education secretary said: "It has been on my mind for some time, this is not an overnight decision.

"I will not be standing as a Conservative candidate at the next election.

"I want to focus on making a difference on the ground on social mobility and I believe I can do that better outside Parliament than inside Parliament. We have seen Parliament gridlocked by Brexit.

"I will continue to represent my community, that heavily voted to remain, on Brexit. I have no doubt that the person following me will also represent our community on Brexit."

Ms Greening, who had been in favour of a second referendum, had been expected to be one of the Tory rebels to oppose Prime Minister Boris Johnson's push to keep a no-deal Brexit on the table.

"It is pretty basic - a job of an MP for me is to be Putney's voice in Parliament," said Ms Greening, who represents a Remain-voting constituency.

"That's certainly what I have sought to do and I will do that today in making sure we pass this Bill hopefully through Parliament on Wednesday.

"My concerns about the Conservative Party becoming the Brexit Party, in effect, have come to pass and my decision is that if I really want to make a difference on the ground and on social mobility, then I need to do that outside Parliament. That's what I'm prepared to do."

Mrs Greening said Conservative leader Mr Johnson was offering a "lose-lose" situation for the country if he called a general election.

During a speech outside Downing Street on Monday evening, Mr Johnson said he did not want a general election, but sources afterwards briefed that an October 14 poll could be on the cards if opposition and rebel Tories voted to block a no-deal Brexit this week.

She said: "I don't believe that the Conservative Party will offer people a sensible choice at the next election in respect of the fact that Boris Johnson is going to offer people a general election that faces them with the choice of a no-deal or Jeremy Corbyn.

"That is a lose-lose general election for Britain. I think a far better way of resolving a way forward on Brexit is to give the British people a direct choice on the different options for Brexit themselves rather than a messy general election which I believe all the evidence suggests will be, yet again, inconclusive on a way forward on Brexit."

Ms Greening was first appointed as the economic secretary to the treasury within the Coalition Government in 2010 and became transport secretary the following year.

She served under David Cameron as international development secretary between 2012 and 2016.

Ms Greening entered Parliament in 2005 as MP for Putney, taking back a traditionally Conservative seat in affluent south-west London that had fallen to Labour.

Mr Johnson faces a showdown in Parliament after he vowed to push for a snap general election if rebel MPs succeed in a bid to seize control of parliamentary proceedings.

Parliament returns on Tuesday after recess, with MPs looking to take control of Commons business to allow them to discuss proposed legislation to block a no-deal Brexit.

Addressing the nation outside Number 10 on Monday, the Prime Minister insisted "I don't want an election, you don't want an election".

But moments later a senior Government source said any bid to "wreck" the UK's negotiating position would prompt a motion for an early election.

The source said Mr Johnson would request a general election on October 14 if the move was successful.

It comes days after former Tory minister Richard Harrington announced he will also stand down at the next general election.

The MP for Watford has been a vocal critic of both Mr Johnson and Theresa May's Brexit strategy.