Leaders of the UK and European Union hardened their positions on Tuesday on whether it is possible to re-open Brexit negotiations, dimming prospects for any breakthrough ahead of a summit of G-7 leaders this weekend.

British Prime Minister Boris Johnson demanded on Monday that the EU re-open Brexit negotiations, scrapping “anti-democratic” provisions for the Irish border that he says would threaten the peace process in Northern Ireland. European Council chief Donald Tusk responded quickly and vigorously, defending the so-called backstop — an insurance policy of sorts meant to avoid a hard border between Ireland and Northern Ireland.

“Those against the backstop and not proposing realistic alternatives in fact support reestablishing a border,” Tusk tweeted Tuesday. “Even if they do not admit it.”

The tweet reinforced the position of Ireland. Irish Prime Minister Leo Varadkar, after a one-hour call with Johnson on Monday, said the Brexit deal wouldn’t be renegotiated.

Johnson is calling for an end to the backstop, which would keep Britain closely aligned with the European customs union if the two sides can’t agree on other ways to prevent the reintroduction of border checks on people and goods moving between Northern Ireland and the Republic of Ireland.

Though the positions of both sides are unchanged, the timing is important.

Since taking office last month, Johnson has pledged Britain will leave the EU on Oct 31, with or without a deal, saying the only way to force European officials to negotiate is to make sure that leaving without an agreement is a real possibility. To that end, he wrote to the EU, underscoring his position only days before he plans to meet with German Chancellor Angela Merkel on Wednesday in Berlin and French President Emmanuel Macron on Thursday in Paris, before traveling to a summit of G-7 leaders this weekend in Biarritz, France.

But he is facing rising criticism of his Brexit strategy at home. A leaked report showed that the British government is preparing for widespread shortages of food, fuel and medicines in the event of a no-deal Brexit.

“Now, of course, our friends and partners on the other side of the Channel are showing a little bit of reluctance at the moment to change their position,” Johnson told Sky News on Monday. “That’s fine. I’m confident that they will — but in the meantime we have to get ready for a no-deal outcome.”

Meanwhile, Jeremy Corbyn, leader of the opposition Labour Party, vowed on Monday to do “everything necessary” to prevent the UK from leaving the EU without a deal. This includes calling a no-confidence vote in Johnson’s government and, if it succeeds, fighting the ensuing general election with a pledge to hold a second public vote on Brexit.

“If MPs are serious about stopping a no-deal crash out, then they will vote down this reckless government,” Corbyn said. “And it falls to the leader of the opposition to make sure no-deal does not happen and the people decide their own future.”

Johnson and Corbyn are fighting for support in an increasingly fractious country where Brexit cuts across traditional party lines.

After a 2016 referendum in which the public voted to leave the EU, then-Prime Minister Theresa May spent more than two years negotiating a Brexit divorce agreement with the bloc. It was repeatedly rejected by Parliament, primarily because of concerns about keeping an open border between Northern Ireland and the Republic of Ireland.