Justine Greening warned Theresa May she will not stay a member of a 'Brexit Party' today as Westminster waits for a new wave of defections to the Independent Group.
The former education secretary said she was staying as a Tory 'for the moment' but said commitment to no deal could push her over the edge.
Her warning echoes criticism of three ex-Tory MPs who quit yesterday warning the Conservatives have been taken over by hardline Brexiteers in the European Research Group.
Chancellor Philip Hammond denied the battle was lost today as he attacked the ERG as a 'small hard core with a hardline view'.
He said he hoped Anna Soubry, Sarah Wollaston and Heidi Allen could in future return to the Tory fold - despite their vow to defeat the Conservatives in their new Independent Group.
Ms Greening is among a raft of MPs on both sides of the Commons facing questions about whether they will join the 11-strong group that broke away from their parties this week.
In other developments of the Independent Group today, a former Labour donor hinted he could fund the group if it gets off the ground.
Justine Greening warned Theresa May she will not stay a member of a 'Brexit Party' today as Westminster waits for a new wave of defections to the Independent Group
Chancellor Philip Hammond denied the battle was lost today as he attacked the ERG as a 'small hard core with a hardline view'.
Mr Hammond said he hoped Anna Soubry, Sarah Wollaston and Heidi Allen could in future return to the Tory fold - despite their vow to defeat the Conservatives in their new Independent Group
The Putney MP has campaigned on social mobility since resigning from Mrs May's Cabinet last year.
Asked if she would defect, she told the Today programme: 'It is something that I have considered, but I have reached a different conclusion for the moment.'
She added: 'I don't think I would be able to stay part of a party that was simply a Brexit party that had crashed us out of the European Union,' she said.
Ms Greening said she had 'not seen the Prime Minister recently' despite asking 'on multiple occasions'.
What happened this week?
Seven MPs resigned from the Labour Party on Monday and set up something called the Independent Group. They were joined by an eigth Labour MP last night.
Three Tory MPs also left their party - warning the Conservatives have been gripped by a right wing, pro Brexit insurgency.
Who is in the group?
Ex-Labour MPs Chuka Umunna, Luciana Berger, Chris Leslie, Mike Gapes, Ann Coffey, Angela Smith, Gavin Shuker and Joan Ryan.
Ex-Tory MPs Sarah Wollaston, Anna Soubry and Heidi Allen.
Is it a political party?
No. It is a small business - technically a limited company - called Gemini A Ltd that was set up Gavin Shuker, one of the MPs, in January.
Can I vote for it?
No. Unless and until it registers with the Electoral Commission, it cannot stand candidates.
Can I donate to it?
Yes. The group is accepting donations through its website theindependent.group. It says donations will support the newly independent MPs.
What does it want?
All the MPs are Remain supporters and most of them have promoted a second referendum to cancel Brexit. They say they want evidence-based policy making to tackle the challenges facing Britain - without being inside the current political parties.
She added: 'I fail to see how we can be successful in the long-term if we are simply a party of Brexit. There will be other parties that stand for Brexit, no doubt, running at the next election.
'If we are not a party that seems credible on economic policy or on opportunity; if we don't reach out to a new generation, then I think we have to accept that we will simply not have the level of support in the country we need to be a credible political party.'
Ex-Tory MP Sarah Wollaston suggested that the Conservative party was 'destroying itself' amid speculation former colleagues could follow her to the new Independent Group.
'I know that there are many colleagues on my side who will be watching carefully and expecting Theresa May to be certain that she is not going to take us out on a no-deal Brexit,' she told Today.
'Certainly I think that a third of the Cabinet, I'm pretty clear, would walk if they were looking at a no-deal Brexit.'
Dr Wollaston said 'the Tory party has changed, I think it has changed irredeemably', adding: 'I would say the Tory party is destroying itself.'
In a later interview, the Chancellor insisted the battle for the Tory Party was not over.
While admitting no deal was 'always a possibility' he attacked the ERG and said they were not in charge.
He said: 'There is a small hard core that have a hardline view and are not likely to be flexible or to compromise.
