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…