LONDON - Britain withholding the 39 billion pound ($50 billion) it promised the European Union as part of its original Brexit plan would not constitute a sovereign default, rating agency S&P Global said on Monday.

Boris Johnson, the leading candidate to be Britain's next prime minister said at the weekend he would withhold the Brexit payment until the EU gave Britain better exit terms, drawing immediate rebuke about a default from France.

"Our (credit) ratings speak to commercial debt obligations," S&P's primary analyst for the UK, Aarti Sakhuja, told Reuters. "The U.K. not paying the 39 billion pound bill would therefore not constitute a sovereign default under our methodology." ($1 = 0.7896 pounds) (1 euro = 0.8928 pounds)

(Reporting by Marc Jones; editing by Thyagaraju Adinarayan) ((marc.jones@thomsonreuters.com; +44 (0)207 542 9033; Reuters Messaging: marc.jones.thomsonreuters.com@reuters.net Twitter @marcjonesrtrs))