French President Emmanuel Macron speaks at the Elysee Palace on May 06, 2019 in Paris, France.

A source close to French President Emmanuel Macron said on Sunday that failing to pay a 39 billion pound ($50 billion) Brexit bill when Britain leaves the European Union would amount to a sovereign debt default.

"Not honouring your payment obligations is a failure of international commitments equivalent to a sovereign debt

default, whose consequences are well known," the source told Reuters.

Boris Johnson, the leading candidate to succeed Theresa May as leader of Britain's Conservative party and therefore its next prime minister, said in a newspaper interview that he would withhold the previously agreed Brexit payment until the EU gave Britain better exit terms.