Justine Greening has said she will quit the Tory party at the next election.
The MP for Putney, Roehampton & Southfields told Radio 4’s Today show that she feels she can make a bigger difference outside of Parliament.
Ms Greening, who is a Remain campaigner, also accused the Tory party of “becoming the Brexit party.”
Boris 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.”
She added: “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, ad 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.
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.”
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.