The British parliament’s vote on Prime Minister Theresa May’s Brexit deal will determine the future of the country, the chief EU Brexit negotiator said on Thursday, insisting that the deal was the only route to secure an orderly withdrawal from the EU.

“If there is no withdrawal treaty, there is no transition, no basis of confidence that we need with the British regarding the future relationship,” Michel Barnier told representatives from cities and regions in the European Union.

Barnier told the gathering of the European Committee of the Regions that it was key now that the withdrawal treaty agreed between Brussels and London be ratified.

“Now is now the moment for everyone to bear their responsibilities. You know the British parliament will give its verdict on this text and on the future relationship in the coming days. It is a vote in which the future of their country is at stake.”

The British parliament is due to vote on Prime Minister Theresa May’s Brexit deal next Tuesday, with the odds looking stacked against May’s government winning that vote.

Many of May’s Conservatives are particularly skeptical about the fallback arrangement, or “backstop,” to guarantee that there is no return to a hard border between Ireland and Northern Ireland, seen as essential to preserving peace.

Supporters of a clean break with the EU say the backstop could leave Britain forced to accept EU regulations indefinitely, or Northern Ireland treated differently from the rest of Britain.

“This backstop,” said Barnier. “We will do everything we can to avoid using it.”

Barnier repeated his view that the Brexit deal that May wants the British parliament to back is the best Britain will get to arrange its orderly withdrawal from the EU.

“The agreement that is on the table — the withdrawal agreement and the agreement on the future relationship — are, in our view, the only and best possible to organize an orderly withdrawal,” he said. (Additional reporting by Daphne Psaledakis.)