The blame game over the likely failure to reach a Brexit deal escalated, with the U.K. accusing German Chancellor Angela Merkel of making an agreement effectively impossible. The pound fell.

According to a British official, Merkel told Prime Minister Boris Johnson that Northern Ireland must remain part of the European Union’s customs union if he wants to secure a divorce agreement. Johnson responded by saying that condition, together with the EU’s unwillingness to engage with his latest proposals, paved the way to a no-deal Brexit.

European Council President Donald Tusk hit back.

.@BorisJohnson, what’s at stake is not winning some stupid blame game. At stake is the future of Europe and the UK 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?

— Donald Tusk (@eucopresident) October 8, 2019

The sniping is more evidence that the U.K. and EU could fail to strike a deal ahead of next week’s summit.

Without that, Johnson has vowed to take Britain out of the EU by the month’s end. But, under the Benn Act, he also is required to seek an extension if he doesn’t have a deal by Oct. 19 — something that may still force him to seek a delay and hold a general election before going back to Brussels again.

EU officials privately believe the British government’s strategy is to pin responsibility for a delay or no-deal Brexit onto them. The bloc has signaled it is willing to permit an extension, seeing a general election as a likely consequence.

The turmoil helped send the pound to the lowest level in a month against the euro on Tuesday.

The EU has always argued that Northern Ireland must remain inside the European customs union in order to ensure there are no physical checks on goods crossing the land border with Ireland. The British government has long insisted Northern Ireland must leave the EU customs union along with the rest of the U.K., to avoid splitting off the region into a different economic zone from mainland Britain.

Late on Monday night, an 800-word text message attributed to someone in Johnson’s office was published on the Spectator magazine’s website, blaming the EU’s refusal to move on the Irish question for the imminent collapse of the talks. The author accepted that an extension was likely, and that Johnson would fight an election promising a no-deal Brexit immediately if he won.

James Slack, a U.K. government spokesman, described the conversation between Johnson and Merkel as a “frank exchange” and denied Tusk’s characterization.

He said it was unacceptable for Northern Ireland to stay in the customs union and that Johnson had told Merkel that the U.K. had made a significant offer and that it was time for the EU compromise. Slack also said that the U.K. hasn’t asked an EU member state to use its veto power to block another delay to Brexit day later this month.

German officials disputed the British account of the call, while others in Europe found it difficult to believe that the typically cautious Merkel would have expressed herself so strongly. Nevertheless, relations were strained by the encounter.

One German official said Merkel was well aware of the blame game Johnson’s office has started, but did not intend to join in trading blows in public.

After the call, Arlene Foster, leader of Northern Ireland’s Democratic Unionist Party, which supports Johnson’s Brexit stance, said accepting the EU’s position would amount to “surrender.”

Hours later, the U.K. government published a report on its preparations for a no-deal exit. Such an outcome could cause shortages of some fruit and vegetables, force drugmakers to stockpile medicines, and require the government to deploy a reserve fleet of 80 oil tankers to maintain fuel supplies.

–With assistance from Tim Ross, Patrick Donahue, Arne Delfs, Helene Fouquet, Jasmina Kuzmanovic, Andra Timu, Dara Doyle and John Follain.