Voters under 40, in particular, are ready to “risk the short-term damage of a Corbyn government” than go ahead with the “calamitous mistake” of leaving the EU, the Tory peer said.
The warning for Theresa May came on the eve of her much-anticipated speech intended to finally set out her detailed strategy for the Brexit negotiations.
And it comes after Sir John Major warned the Government was in such a state that Labour would be “15-20 points” ahead under any other leader.
Lord Heseltine – who, like Sir John, has called for a further referendum to stop Brexit, if necessary – said: “I hate the idea of Jeremy Corbyn in power.”
But he added: “There are increasing numbers of people, particularly the young people under 40, who today think that Corbyn is an alternative they can live with.
“There are Conservatives who feel so strongly about the European issue they would rather risk the short-term damage of a Corbyn government – and let’s not underestimate that – than to see Britain make this calamitous mistake of leaving Europe”.
The former Deputy Prime Minister, speaking to BBC Radio 4’s Today programme, also praised the “very brave” Tory MPs willing to defy Ms May to prevent a hard Brexit.
They had been derided as traitors and saboteurs, but were following their conscience which “tells them they have to vote for what they believe to be right for Britain”.
Calling for Parliament to be ready to vote down any Brexit deal, Lord Heseltine said: “The fact is that the Prime Minister has no majority for the sort of deal that is being talked about.”
However, Labour has insisted it will not stop Brexit, while leaving open the option of voting against the final deal with the purpose of sending the Prime Minister back into the talks.
Meanwhile, Chris Grayling, the Transport Secretary, denied that Ms May would be “about cherry-picking” the best bits of EU membership.
He said the Government accepted the UK would not be able to “have everything that we might like” from the EU after Brexit and that some of the current strong rhetoric would “come out in the wash” during the course of the negotiations.
“The Prime Minister will recognise that it is not about cherry-picking, we can't have everything that we might like to have because we are leaving,” Mr Grayling said.
But he also predicted a softening in Brussels' stance, adding: “I talk to my European counterparts all the time.
“What I experience, what my fellow ministers experience, is a genuine desire in other European capitals to maintain a good, close relationship.”
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