“I think we can get through this, this is a great, great country, the UK, we can easily cope with a no-deal scenario,” Johnson insisted in Biarritz, as he made his debut on the international stage as prime minister with a series of bilateral meetings with world leaders including Donald Trump, the EU council president Donald Tusk and Indian prime minister Narendra Modi.

Johnson said preparations for no deal were being ramped up to help secure an agreement, but also “so that if and when we are forced by the obduracy by our European friends to come out on 31 October without a deal that things are as smooth as they can possibly be”.

Johnson claimed food shortages – one of the risks outlined in the leaked Operation Yellowhammer documents on no-deal planning – were “highly unlikely”, and offered a “guarantee” that patients would be able to access medicines unhindered.

The prime minister said that in the event of no deal the UK would withhold much of the £39bn financial settlement agreed by Theresa May – and insisted it was up to the EU27 to avert that eventuality.

“If we come out without an agreement it is certainly true that the £39bn is no longer, strictly speaking, owed,” he said “There will be very substantial sums available to our country to spend on our priorities. It’s not a threat. It’s a simple fact of reality.”

During the Conservative leadership campaign, Johnson suggested the entire £39bn would be retained in the hope of using it as leverage to win a better future trading relationship from the EU27. But Downing Street appears to have conceded that legal obligations for past liabilities may mean up to a quarter of it may still have to be paid.

Johnson is battling to keep alive the prospect of striking a reworked exit deal with the EU27 in time for Britain to leave by the Halloween deadline, which he has made it a “mission” of his government to meet.

But with just a week until MPs return to Westminster, preparing to seize any opportunity to bind his hands, Johnson has so far presented no detailed plan.

After Johnson met Tusk on the sidelines of the G7 summit on Sunday, an EU official said, “nothing really happened”. “It was essentially just a reconfirmation of of the views of both sides. There were no new substantive elements from any side, and obviously not from the UK side,” the official said.

“What we ideally would have been hoping for and looking for are new ideas that unblock this situation,” the European official said. “So we are waiting … We need input from their side.”

Meanwhile, it emerged this weekend that Downing Street has sought legal advice from the attorney-general Geoffrey Cox about the possibility of shutting down parliament from September.

Asked about the issue on Sunday, a senior government official said, “Number 10 commissions legal advice on a whole range of issues, but the PM is clear that he is not going to stop MPs debating Brexit”.

Johnson’s parliamentary opponents appear unable to present a united front, however. The shadow trade secretary, Barry Gardiner, accused the Liberal Democrat leader, Jo Swinson, of being “extremely petulant” on Sunday, after she raised doubts about whether Jeremy Corbyn was the right person to lead a caretaker government to prevent a no-deal Brexit.

Gardiner told Sky News that the Labour leader was offering a “failsafe” way of achieving the Lib Dems’ Brexit aims, but he said Swinson had concluded, “oh well, we are not going to cooperate if Jeremy Corbyn is going to be the person who does it”.

Labour has suggested it could table a vote of no confidence in Boris Johnson’s government – but is unlikely to do so immediately after MPs return from summer recess unless it is convinced Tory rebels are ready to back it.

The former chancellor Philip Hammond revealed the extent of the bad blood between Downing Street and Conservative moderates on Sunday, as he wrote to the PM demanding an apology for briefings that suggested the Yellowhammer leak came from former ministers.

Hammond said that it had since emerged the document was dated to August, and thus could not have been leaked by one of the moderates dispatched to the backbenches in Johnson’s summer reshuffle.

A government official said Johnson would respond, “in due course”.

On Sunday, Johnson claimed the Brexit mood music had improved significantly over recent days; but it remained “touch and go” whether a deal was achievable.

Throughout the summit in Biarritz, Johnson has sought to stress the UK’s determination to remain internationalist – and to distance itself from Trump’s White House on some questions.

At a dinner of G7 leaders on Saturday night, which sources said was occasionally testy, Johnson sided with Germany, France and others against the US president’s argument that Russia should be readmitted to the group.