Boris Johnson has insisted he is not bluffing over his commitment to take the UK out of the European Union on October 31 – with or without a deal.
The Tory leadership frontrunner said the EU had to "look deep into our eyes" and realise that the UK was prepared to walk away.
His campaign received the support of Home Secretary Sajid Javid, who said Mr Johnson was "better placed" than Jeremy Hunt to "deliver what we need to do at this critical time".
In an interview with the Sunday Telegraph, Mr Johnson accused Theresa May of presiding over a "diet of miserablism" and a "computer says no" approach in government.
Asked if his commitment to the October 31 deadline was a bluff, Mr Johnson said: "No ... honestly. Come on. We've got to show a but more gumption about this."
He added: "We were pretty much ready on March 29. And we will be ready by October 31.
"And it's vital that our partners see that. They have to look deep into our eyes and think 'my god, these Brits actually are going to leave. And they're going to leave on those terms'.
"Everybody who says 'I can't stand the idea of a no-deal Brexit', what they really mean is actually they don't want to leave at all."
In a hustings in Cardiff on Saturday night, Mr Johnson said: "If we had to come out on WTO (World Trade Organisation) terms, I really think this country has the versatility and the creativity to get through it and prosper and thrive."
Mr Hunt also insisted he was not bluffing about being willing to walk away without a deal, although he told the Sunday Telegraph it was "not the most secure way of guaranteeing Brexit" because MPs would try to block it.
He also promised an overhaul of treason laws to "make the punishment fit the crime", with life sentences for Britons who join jihadist groups fighting against UK forces.
"We must be able to properly punish those who betray our country," he said.
"Betraying Britain by supporting terrorism is one of the most serious crimes a person can commit."
The move is intended to address concerns that fighters returning to Britain from war zones overseas could escape prosecution.
The Sunday Times said Mr Javid's support for Mr Johnson comes ahead of a speech on Tuesday in which he will call for an emergency budget to prepare for a no-deal Brexit.
Mr Javid, who is widely believed to have his sights set on becoming chancellor under the new prime minister, will say: "Trust in our democracy will be at stake if we don't make October 31 a 'deal or no deal' deadline.
"To prepare that, we are agreed on the need for ramped-up no-deal preparations, including a budget."