Former Foreign Secretary Boris Johnson is stepping up his campaign to be Britain's next prime minister by challenging the European Union over Brexit terms.

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.

Johnson told the newspaper he is the only Conservative Party leadership contender who can triumph over the Labour Party led by Jeremy Corbyn and Nigel Farage's Brexit Party.

He is threatening to leave the EU on the Oct 31 deadline even if there is no deal in place.

The contest for leadership of the Conservative Party officially begins Monday. The post was vacated Friday by Prime Minister Theresa May.