Johnson told the Sunday Times he would refuse to pay the agreed-upon 39 billion-pound ($50 billion) divorce settlement unless the EU offers Britain a better withdrawal agreement than the one currently on the table.
The contest for leadership of the Conservative Party officially begins Monday. The post was vacated Friday by Prime Minister Theresa May, who will serve as a caretaker until a new leader is chosen and moves into 10 Downing Street.
The party expects to name its new leader in late July.
Johnson, the early frontrunner in a crowded field, told the newspaper he is the only contender who can triumph over the Labour Party led by Jeremy Corbyn and Nigel Farage’s Brexit Party.
Johnson is a hard-line Brexit advocate who vows to take Britain out of the EU on the Oct. 31 deadline even if there is no deal in place.
He and other contenders say they can get better terms from EU leaders in Brussels than the deal that May agreed to but was unable to push through Parliament. Those failures led to her decision to resign before achieving her goal of delivering Brexit.
But EU officials have said they are not willing to change the terms of the deal May agreed to.
One of Johnson’s main rivals for the post, Environment Secretary Michael Gove, continued to be sidetracked Sunday by questions about his acknowledged cocaine use when he was a youthful journalist.
He told BBC Sunday that he was “fortunate” not to have gone to prison following his admission of cocaine use. He said he was “very, very aware” of the damage drugs can cause.
Nominations for the leadership post close Monday afternoon.