The Bank of England said in November that a chaotic exit would see the economy shrinking by 8 per cent, property prices plunging almost a third and the pound losing a quarter of its value to below parity with the dollar.
Johnson's pledge, in an interview with the Sunday Times, brought him the endorsement of self-styled "Brexit hard man" Steve Baker, a key member of the anti-EU caucus of Tory MPs. Only Johnson could stop the Conservatives from losing votes to Nigel Farage's Brexit Party as they did in last month's European elections, Baker said.
"I am going to put my complete faith in Boris Johnson," Baker wrote on Twitter. "Unless we deliver a Brexit worth having in the opinion of Brexit Party voters then we will suffer a Jeremy Corbyn government with all the horrors that would mean for our prosperity and our wellbeing."
Foreign Secretary Jeremy Hunt, another contender, also warned against the dangers of a general election, but said the only way to avoid one is to exit the EU with a deal. Any prime minister who tries to leave without an agreement would find themselves forced to call a national vote, he said as he made a veiled attack on Johnson's suitability for the top job.
"What a wise prime minister will do is take decisions on the basis of the choices they have in front of them," Hunt told Sky News. "What an unwise prime minister will do in this situation is something that precipitates a general election. If you say October 31 is a deadline come what may, and then Parliament blocks no deal, the only way you can deliver that promise is to have an election."
If an election is held before the UK leaves the EU, the centre-right vote would be split between the Conservatives and the Brexit Party and Labour would "come through the middle" to win, Hunt said. Both Hunt and Gove refused to rule out extending Brexit beyond the end of October to allow an agreement to be reached.
Hunt, who is pitching himself as an experienced negotiator who can find a way through the Brexit impasse, also said German Chancellor Angela Merkel told him when they met last week that the EU is open to new talks "if we take the right approach."
A common theme among the contenders was that future negotiations should be carried out by politicians rather than officials. Gove, Hunt and Johnson all said they would take charge of talks with the EU, sidelining the officials who led the talks under May.
Johnson said withholding the £39 billion-pound financial settlement will be a "great solvent and a great lubricant" in forcing the EU to offer a good deal to the UK. But the experience of Greece, when it threatened to default on some €250b of loans to the bloc's crisis-fighting fund in an attempt to extract concessions from its European peers, shows the EU might not be that easily lubricated.
Home Secretary Sajid Javid, who won the support of Scottish Conservative Party leader Ruth Davidson for his campaign, said if he succeeds May he will provide funding to Dublin to pay for solutions to the impasse over the border between Northern Ireland and the Irish Republic.
The border issue can be solved but "you need co-operation on both sides of the border to make it happen," Javid told Sky News. It is "morally right" that the UK should pay for the alternative arrangements, he said.
Javid also pledged to slow the pace of UK debt reduction to allow for more investment in education.
- Bloomberg