The Brexit-dominated battle is expected to conclude by the end of next month, with former foreign secretary Boris Johnson the clear frontrunner after topping the first round of votes this week.
Johnson said in an interview that he would refuse to pay Britain's divorce settlement until the EU agreed better withdrawal terms.
The former London mayor also signalled he would scrap the unpopular backstop and try to settle the border issue once London and Brussels were negotiating their future relationship.
“Our friends and partners need to understand that the money is going to be retained until such time as we have greater clarity about the way forward,” he told The Sunday Times.
“In getting a good deal, money is a great solvent and a great lubricant,” Johnson added, in his first newspaper interview since launching his bid to become Britain's next premier.
A source close to French President Emmanuel Macron told AFP that a refusal by Britain to pay its Brexit bill would be viewed as the country defaulting on its debts.
“Not honouring payment obligations is a breach of international commitment equivalent to a default on its sovereign debt, with the consequences that we know,” the source said.