Ahead of a titanic Commons showdown over the prime minister's Brexit plans, former cabinet minister Justine Greening said he had transformed the Conservatives into a "Brexit" party, which was "narrowing down its appeal" to the public.
Threats of deselection from Downing Street had not worked, the ex-international development secretary said, as she vowed to support a rebel bill tabled by Labour's Hilary Benn to force Mr Johnson to seek an extension to the Brexit deadline.
Ex-foreign office minister Alistair Burt also confirmed he would not stand in the next poll after more than 18 years in the Commons, as he prepared to rebel against the government.
More than a dozen Tory rebels have said they will back efforts to thwart no deal, despite Mr Johnson's attempt to scare off a rebellion by letting it be known he would push for a snap election if MPs win a crunch vote to seize control of parliamentary proceedings on Wednesday.
No 10 has also made it clear that any Tory MPs would support such a move would have the whip removed and could be banned from standing as Conservative candidates in a looming poll.
However Ms Greening, who supports The Independent's campaign for a Final Say referendum, told the Today programme: “I have sent a letter to the prime minister saying I will not be standing as a Conservative candidate at the next election."
She joins a string of Tories who have vowed not to stand again - making them immune to threats from the Tory whips - including pro-EU MP Guto Bebb, former business minister Richard Harrington and prominent rebel Sir Oliver Letwin.
In a speech outside Downing Street on Monday, Mr Johnson said he did not want a general election, but sources said that a poll could held on 14 October if opposition and rebel Tories voted to block a no-deal Brexit this week.
Ms Greening 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."
The Putney MP said her party was "narrowing down its appeal", a point highlighted by No10's threats to sack long-serving MPs if they voted against the government on Brexit.
She added: "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."
Mr Burt, a long-serving MP and minister, told members in his North East Bedfordshire seat: "It has become clear that I have a fundamental, and unresolvable disagreement with our party leadership on the manner in which we leave the EU, and the consequences going forward of doing so.
"This is very likely to be at the root of the next election, and I believe it is unfair of me to present you with a conflict of interest between my views and those of the party at an election, even if current circumstances do not result in my having the whip in parliament removed."
Elsewhere, vteran Tory MP Keith Simpson also vowed to stand down at the next general election, although he said the decision was "nothing to do with Brexit".
The MP for Broadland in Norfolk suggested he will vote with the government during a series of crunch votes.
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