BRUSSELS — European policymakers said Thursday that a new Brexit proposal from British Prime Minister Boris Johnson was unworkable, heightening the prospects of a chaotic British departure from the European Union within weeks.

Although leaders, politicians and negotiators did not dismiss Johnson’s plan out of hand, they made clear the current offer would not win support from the 27 countries that need to sign off on any withdrawal deal, and they were downbeat about it serving as the basis for serious negotiations.

British negotiators plan to visit Brussels again Friday.

The two sides remain far apart without much time to resolve their differences. Britain is scheduled to leave the European Union on Oct. 31. Parliament passed a law demanding that the prime minister request an extension if a withdrawal deal has not been agreed by mid-October. But Johnson has continued to insist that Britain will leave with or without a deal at the end of the month.

Johnson’s most substantive plan so far, presented to E.U. leaders Wednesday, tries to address issues in Northern Ireland — the major barrier to a withdrawal agreement. But his written proposal crosses several E.U. red lines. Some policymakers assessed that it would be more damaging to the European Union than the economic and political instability that could come with an abrupt “no-deal” Brexit without any transition period to buffer the way.

“Safeguarding peace and stability on the island of Ireland, protection of citizens and E.U.’s legal order has to be the main focus of any deal,” read a statement from the European Parliament’s Brexit oversight committee. “The U.K. proposals do not match even remotely what was agreed as a sufficient compromise” by Johnson’s predecessor, Theresa May.

The European Parliament can veto any deal approved by the leaders of E.U. member states, who make up the European Council.

Many European leaders appear to be deferring — for now — to Ireland because of how directly it would be affected by the terms of a Brexit withdrawal.

“There are elements of this proposal that simply will not be part of any deal,” Irish deputy leader Simon Coveney said Thursday.

European Council President Donald Tusk said he had spoken to both Ireland’s and Britain’s leaders Thursday. “We stand fully behind Ireland,” he tweeted. To Johnson, he said, “We remain open but still unconvinced.”

There was similar skepticism from the European Commission, the European Union’s executive body.

“There are problematic points in the U.K.’s proposal, and further work is needed,” said European Commission spokeswoman Natasha Bertaud.

Although British Brexit Secretary Stephen Barclay had admonished that the ball was in the European Union’s court, Bertaud emphasized, “This work is for the U.K. to do, not the other way around.”

“We are not going to be the ones left holding the bag, the ball or any other kind of object,” she added — reflecting a fear on the European side that Johnson is setting them up to take the blame for a Brexit failure.

The biggest disagreement centers on what to do about the border between Northern Ireland, which is leaving the European Union along with the rest of Britain, and the Republic of Ireland, which is staying.

Since the 1998 Good Friday Agreement that ended decades of sectarian violence, people and goods have been able to pass from one side of the border to the other without any checks. The European Union has maintained that keeping the border open and invisible is critical to ensuring that violence does not flare again.

But Johnson’s proposal would lead to some form of customs controls, setting off nervousness on the island of Ireland.

“Our objective is very clear — we don’t want to see any customs posts between north and south, nor do we want to see any tariffs or restrictions on trade between north and south,” Irish Prime Minister Leo Varadkar said in a Thursday news conference. “They were all abolished in the 1990s, and we don’t want to go back to that.”

He said he appreciated Johnson’s assurances that there would be no new customs infrastructure. But Varadkar said those assurances seemed to be contradicted by Johnson’s written proposal.

Also making the Europeans anxious: Johnson’s plan would require the European Union to maintain a loosely controlled border with a neighbor that already has said it wants to go its own way on taxation and regulation. E.U. businesses fear such a step could cost them by opening the door to smuggling.

“Brexit is not a game or a bargaining process. We are not ready to do everything in order to have a deal,” French European Affairs Minister Amélie de Montchalin told the CNews broadcaster. “The United Kingdom cannot become a fiscal paradise nor an exporter of goods that do not respect the health and safety rules established by the European Union.” 

If there is any chance for a deal at a summit of E.U. leaders that begins Oct. 17, serious negotiations would have to begin immediately, policymakers say. But those talks don’t appear imminent.

The prospects for approval of Johnson’s plan by the British Parliament are also uncertain. Johnson seems to have won the backing of some hard-line Brexiteers who voted against previous Brexit proposals. But he has been hostile toward opposition lawmakers whose support he also needs.

Labour Party leader Jeremy Corbyn slammed Johnson’s government Thursday for seeking “a Trump-deal Brexit that would crash our economy” and undermine workplace rights and environmental standards.

Quentin Ariès contributed to this report.