LONDON (Reuters) – Britain’s withholding of 39 billion pounds ($50 billion) it promised the European Union as part of its original Brexit plan would not constitute a sovereign default, the top credit-rating agencies say, though lawyers warn it would be highly damaging.

Boris Johnson, the leading candidate to be Britain’s next prime minister, said at the weekend that he would retain the Brexit payment until the EU gave the UK better exit terms.

The comments drew an immediate rebuke from a source close to France’s President Emmanuel Macron, who warned it would be “equivalent” to a debt default. But the big rating agencies said it would not be one by their definitions.

“Our ratings speak to commercial debt obligations,” S&P’s primary analyst for the UK, Aarti Sakhuja, told Reuters. “The UK not paying the 39 billion-pound bill would therefore not constitute a sovereign default under our methodology.”

A spokesman for Fitch said it wouldn’t be a default in its view, either. Moody’s took a similar approach.

“Sovereign ratings speak solely to the risks faced by private investors and not the risks borne by official sector entities,” it said in 2015 when Greece was negotiating better repayment terms on its euro zone and IMF bailout loans.

There are still likely to be implications, however. The agencies have warned Britain would almost certainly see its rating cut again if it tumbled out the EU without a transition deal – something withholding the divorce cash could imply.

S&P believes the EU would not commence discussions on a fresh relationship with the UK unless London commits to making good on its obligations. Lawyers, meanwhile, warn that it could set up a clash in the international court in the Netherlands.

“I am quite clear that we (Britain) have to pay what is owed up to the date we leave as a matter of law,” said Ros Kellaway, the head of Eversheds Sutherland’s Competition, EU and Trade Group.

“So there would be a default (if payments were not made) and we could end up in front of the international court in The Hague, That is quite straightforward.”

It also would leave the other EU member states with a big hole in their finances, she said. “One can hardly bear to imagine what this would do to relations.”

($1 = 0.7896 pounds)

(1 euro = 0.8928 pounds)