Veteran Europhile Ken Clarke said more “mainstream” Conservatives could be “driven out” if “hardline right wing nationalists” decide who succeeds Theresa May.
Three pro-EU Conservative MPs - Heidi Allen, Anna Soubry and Sarah Wollaston - quit the party last month to join the Independent Group, claiming that their former party was being forced to the right.
Ahead of a vote in the Commons this week which is likely to see the Prime Minister’s Brexit deal defeated a second time, Mr Clarke said Article 50 should be extended until December 2021, keeping the UK in the EU for almost two years more - and he warned that negotiations on the future relationship between London and Brussels could take up to five years.
“It’s this daft promise to be bound by the result of the referendum that has caused the present confusion and in my party has made the extreme right, the hardline right wing nationalists, more influential and powerful than they’ve been before.
“I think Theresa’s got to make sure they don’t take over the party. The party's got to make sure it isn’t taken over by this essentially breakaway party that exists within it.”
A Brexiteer former minister this week told Scotland on Sunday that a new party leader should be given time to “bed in” ahead of the next general election, scheduled for 2021.
Mr Clarke added: “Theresa thinks it’s absolutely essential to keep negotiating with them all the time, and making concessions to them and then trying to go off to Brussels to tell them they’ve got to give in to the ERG.
“The ERG are making desperate attempts to get rid of her. They’ve launched on coup already, they can’t wait to get rid of her because they think one of their members will replace her.
“If they were successful I don’t know what Conservatives like me would do… obviously theres a risk that more people will feel driven out like the three excellent MPs that have gone off to join the Independent Group. I don’t agree with them, but I think it’s extremely sad that we’ve lost them.”
With MPs set to be given a vote on whether to delay Brexit in the event that Mrs May’s deal is defeated this week, Mr Clarke said: “I think we need to extend Article 50, if we don’t get an agreement, until the end of 2021.
“I think we would do well to finish by then. I think it could take four or five years to do a full, proper negotiation.
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“We can’t conduct it like this. We’ve got to begin by some indicative votes in parliament that established what there is a consensus and a potential majority for.
“We can’t carry on in this week by week crazy riven atmosphere of rather hysterical overreactions and imagined plots buried in the Irish backstop. The other Europeans negotiating with us are quite baffled.
“What are they supposed to do? They always thought we were a model democracy with reasonably stable, sensible governments.
“Now they find they're trying to negotiate with people who have no idea what they want, and if they do know what they want, don't seem able to get parliament to vote for it.”