The Conservative leadership hopeful Jeremy Hunt has claimed Angela Merkel would be willing to renegotiate the UK’s Brexit deal and possibly the Irish border arrangements opposed by Eurosceptics.

The foreign secretary said he had spoken to the German chancellor on the sidelines of D-day commemorations last week and she had indicated that the EU was “willing to negotiate on the package” if a new prime minister had the “right approach”.

His claim flies in the face of repeated assertions from Brussels that the withdrawal agreement cannot be reopened to change the Irish backstop, which Eurosceptics reject because they fear it may keep the UK in a customs union indefinitely.

Speaking to Sky’s Ridge on Sunday programme, Hunt said Merkel had told him Brussels “would look at any solutions” the UK put forward to solve the Irish border issue.

In apparent criticism of the leadership frontrunner, Boris Johnson, Hunt said an “ultra hardline person” would be met by “an ultra-hardline response” from the EU.

Johnson told the Sunday Times he would withhold the UK’s payments to the EU until a better deal could be struck. He suggested the settlement of £39bn over the coming decades would be paid only when there was “greater clarity” about the way forward.

“I always thought it was extraordinary that we should agree to write the entire cheque before having a final deal,” Johnson said. “In getting a good deal, money is a great solvent and a great lubricant.”

Johnson’s claim that he would play hardball with the EU over money appears to have won him a string of new Eurosceptic backers, including Steve Baker, Priti Patel, Chris Grayling, James Cleverly and Owen Paterson.

At the same time he has convinced a growing number of Tory moderates to support him, including the former minister Chloe Smith and James Brokenshire, the communities secretary, who is a close ally of Theresa May.

In his Sunday Times interview, Johnson said it was only a “very, very small possibility” that the UK would have to leave the EU without a deal, and he stressed his determination to get an agreement. But he added: “People must be ready for the possibility of some disruption.”

Sajid Javid, another Tory leadership hopeful, joined fellow candidates in saying he could negotiate a better deal than May but that he would be prepared to leave on 31 October without one. “If I have to choose between no deal and no Brexit, I would pick no deal,” he said.

He insisted his time as home secretary had shown it was possible to have an invisible border between the Republic of Ireland and Northern Ireland, saying: “You don’t need a magic solution for this. The solution exists.”

“We’ve done the homework on this,” he told the Ridge programme, pointing out that he oversaw the Border Force in the Home Office. “I will change the dynamic and I will do that by offering the money to pay for the border. It is justified that we do that because, economically, if that unlocks a deal we will have a mini economic boom in this country if we get a deal and that will pay for that.”

Javid’s focus in the interview was a new pledge that he would slowing the pace of debt reduction to fund a multibillion-pound spending spree, releasing up to £25bn a year for spending priorities including a funding boost for education.

Michael Gove, another leading candidate in the race, was less clear than Johnson and Javid that he would stick to the exit date of 31 October, saying he would want to extend article 50 if Britain was close to signing an exit deal.

“If we’re 95% of the way there on 31 October, would it really make sense to junk that progress and to say ‘tell you what, we’re leaving without a deal anyway’?” he said on the BBC’s Andrew Marr Show.