As speculation grew that Britain might have to delay its exit from the bloc beyond the March 29 deadline, European Commission President Jean-Claude Juncker said "we are checking with Downing Street what the clarifications could amount to" that might help May get her Brexit deal approved by Britain's Parliament next week.
But, Juncker added: "They should not be confused with a renegotiation."
An EU official said the bloc and the British government "are in contact at all levels ... to make sure that the deal goes through." The official spoke on condition of anonymity to discuss the diplomatic talks.
The two sides are discussing possible reassurances to help persuade reluctant British lawmakers to back the deal in a vote Tuesday in Parliament.
Britain and the EU reached a hard-won Brexit deal in November, but the agreement has run aground in the British Parliament. May postponed a vote on the deal in December to avoid a resounding defeat, and there are few signs the deal has picked up support since then.
May promised to seek further guarantees from Brussels on the most contentious issue, the status of the Ireland-Northern Ireland border.
The EU is adamant that the legally binding 585-page withdrawal agreement can't be reopened, but EU officials are looking for diplomatic wording that could sway reluctant U.K. lawmakers.
The Brexit agreement aims to guarantee Britain's smooth departure from the bloc, with a long transition period to adapt to the new situation and negotiate a permanent trade agreement.
Without a deal, Britain faces an abrupt break from the EU on March 29, and there are fears it could involve chaotic scenes at borders, ports and airports. Businesses and people in Britain would face uncertain weeks and months as they try to find out what the uncharted future would bring.
"I don't like the prospect of a 'no deal.' It would be a catastrophe," Juncker said on a visit to Romania, as the nation took over the EU's rotating presidency.
Most British lawmakers also oppose the prospect of leaving the EU without a deal. But many also dislike May's agreement, which has displeased both sides of Britain's Brexit divide.
Many lawmakers who back leaving the EU say it leaves the U.K. tethered to the bloc's rules and unable to forge an independent trade policy, while pro-Europeans argue it's inferior to the frictionless economic relationship Britain that currently enjoys as an EU member.
A defeat in Tuesday's vote will leave Britain staring at an exit without a deal 10 weeks later, unless the country's feuding politicians can quickly agree on another plan. This week, British lawmakers passed an amendment forcing May's government to come back to Parliament with a new proposal within three working days of the deal being rejected.
But with no majority in Parliament for any single alternate course, there is a growing chance that Britain may seek to postpone its departure date while politicians work on a new plan.
Extending the deadline would require the EU's approval.
The EU official who spoke on condition of anonymity said such an extension was permitted by the EU's governing treaty, and that a defeat in the Parliament would put the Brexit ball back in Britain's court.
British Foreign Secretary Jeremy Hunt warned Friday that the country would face "Brexit paralysis" if lawmakers rejected the deal next week.
"And Brexit paralysis ultimately could lead to no Brexit," he told the BBC.
Jill Lawless reported from London.
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