Leaving the EU will “take our economy off the cliff and make Black Wednesday look like a picnic”, he said – leading to angry voters wreaking their revenge on the Tories.
Mr Chapman, who was chief of staff to Brexit Secretary David Davis, also revealed that two cabinet ministers had expressed interest in his call for a new political party to fight EU withdrawal.
“Two people in the Cabinet, a number of people who have been in Conservative Cabinets before now – better Cabinets I might say than the current one – and a number of Shadow Cabinet ministers have also been in touch,” he said.
“They are not saying they are going to quit their parties, but they are saying they understand there is an enormous gap in the centre now of British politics.”
Mr Chapman, who left Whitehall earlier this year for a public relations firm, has called for a new centrist party called the Democrats to be set up.
“The Conservative brand has been damaged to such an extent that the party won’t ever get a majority again,” he told BBC Radio 4’s Today programme.
“There are times in our national life when you have to put your nation before your country and the hard Brexit plan that Mrs May is pursuing is going to take our economy off the cliff and make Black Wednesday look like a picnic.”
Cabinet ministers “contemplating this disaster” knew they would never again be in power if Brexit went ahead as planned.
“Remember that 60 per cent of Tory MPs were for Remain and now they are paralysed – they won’t say what they think,” Mr Chapman added.
The former aide – who also worked for George Osborne – denied the former Chancellor was behind the push for a new party, saying: “Not at all. I have not spoken to George about this.”
But Jacob Rees-Mogg, a leading Brexit-backing Conservative MP, poured scorn on the idea of cabinet ministers quitting to try to halt EU withdrawal.
“About 70 per cent of people just want Brexit to be delivered, however they voted a year ago, because they just want democracy,” he said.
Mr Chapman was political editor of the Daily Mail before leaving to work for Mr Osborne and then Mr Davis, but has now renounced the Conservative party.
He has said Boris Johnson should “be in jail” for the broken Brexiteer promise to deliver an extra £350m-a-week to the NHS.
And he has claimed the hundreds of extra lorry bays required to deal with extra post-Brexit border checks would “mean compulsory purchase and demolition of lots of Brexit voters’ houses”.
None of the thousands of extra customs officers that would be needed if Britain leaves the EU customs union had been hired, he said.
However, there are enormous obstacles in the way of forming a new political party, particularly after Labour bounced back under Jeremy Corbyn’s leadership at the June election.
Talk of scores of moderate Labour MPs potentially defecting has evaporated and pro-EU Conservative MPs have dismissed the idea of a breakaway party.
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