A deal for Britain to leave the European Union might be less likely after German Chancellor Angela Merkel told British Prime Minister Boris Johnson Tuesday his alternative to the Irish backstop would not survive an EU vote.

A British government source briefing journalists on a call between Johnson and Merkel said the chancellor "made clear a deal is overwhelmingly unlikely and she thinks the EU has a veto on us leaving the customs union."

"Merkel said that if Germany wanted to leave the EU they could do it no problem but [Britain] cannot leave without leaving Northern Ireland behind in a customs union and in full alignment forever," the government source said.

The call between Merkel and Johnson was reported by BBC News, Sky News and The Guardian.

Negotiations are still going in Brussels, but Merkel told Johnson on the call Tuesday they're close to breaking down.

Johnson has previously vowed to leave the EU by Oct. 31 with or without a withdrawal agreement -- but he has also said he will abide by the Benn Act, which would force him by law to seek a delay if a deal isn't present by Oct. 19. A legal request to force him to seek an extension was declined Monday by Scotland's Court of Session.

The Irish backstop would keep Northern Ireland, part of Britain, in the EU single market and customs trade as an insurance mechanism to prevent a hard border between it and Ireland. Johnson's proposal called for customs stations on both sides of the border to screen goods between Britain's Northern Ireland and the EU's Ireland -- mostly electronically to prevent physical border checks. EU and Irish officials have effectively rejected that proposal.

Johnson told Merkel on the call Tuesday his administration's offer was "reasonable" but blamed the EU for not having "any desire for negotiation." He added that some in the EU are rooting for a second Brexit referendum, but said that won't happen.

"It was a very useful clarifying moment in all sorts of ways," the government source said. "If this represents a new established position then it means a deal is essentially impossible not just now, but ever."

European Commission President Donald Tusk criticized Johnson's position Tuesday.

"What's at stake is not winning some stupid blame game, he tweeted. "At stake is the future of Europe and the U.K. as well as the security and interests of our people. You don't want a deal, you don't want an extension, you don't want to revoke, quo vadis?"