Britain's Labour Party said a short delay would force lawmakers to decide between accepting a deal they already rejected or leaving without a deal at all.
Prime Minister Theresa May has asked for a three-month delay to Brexit Wednesday to buy time to get her twice-rejected departure deal through parliament, but the request faced immediate resistance from the European Commission.
May's initiative came just nine days before Britain is formally due to leave the European Union and marked the latest twist in more than two years of negotiations that have left British politics in chaos and the prime minister's authority in tatters.
After the defeats in parliament opened up the possibility of Britain leaving the EU without a deal and a smooth transition, May said she remained committed to leaving "in an orderly manner" and wanted to postpone Brexit until June 30. The pound fell sharply as May requested her extension.
Her announcement prompted uproar in Parliament, where the opposition Labour Party accused her of "blackmail, bullying and bribery" in her attempts to push her deal through, and one prominent pro-Brexit supporter in her own Conservative Party said seeking a delay was "betraying the British people."
Britain voted in 2016 to leave the EU in a referendum 52 to 48 percent. The decision has split the country, opening up divisive debates over the future of the economy, the nation's place in the world, and British identity itself.
Some European leaders have welcomed May's extension plan, with Germany saying a disorderly British departure would be in nobody's interest.
But a European Commission document seen by Reuters said the delay should either be several weeks shorter, to avoid a clash with European elections in May, or extended at least until the end of the year, which would oblige Britain to take part in the elections. May said it was not in Britain's interests to take part in European elections.
"Any extension offered to the United Kingdom should either last until 23 May 2019 or should be significantly longer and require European elections," the EU document said. "This is the only way of protecting the functioning of the EU institutions and their ability to take decisions."
The document also said the EU should offer Britain just one extension as multiple delays would leave the bloc in limbo. European Commission President Jean-Claude Juncker had warned May against requesting a Brexit delay beyond the European elections unless Britain takes part, an EU spokeswoman said.
Britain must decide by April 11 if it will participate in the May 23 European election, creating an effective deadline for parliament to pass May's deal for an orderly Brexit.
Nearly three years after the United Kingdom voted to leave the European Union, British politicians are still arguing over how, when or even if the world's fifth largest economy should leave the bloc it first joined in 1973.
When May set the March 29 exit date two years ago with the formal Article 50 divorce papers, she declared there would be "no turning back." However, British Parliament's refusal to ratify the withdrawal deal she had agreed to with the EU has thrown her government into crisis.
On Wednesday, May wrote to European Council President Donald Tusk to ask for the delay.
"As prime minister, I am not prepared to delay Brexit any further than the 30th of June," May told a rowdy session of parliament. "I have therefore this morning written to President Tusk, the president of the European Council, informing him that the UK seeks an extension to the Article 50 period until the 30th June."
She said she planned to ask parliament to vote a third time on her departure deal, which lawmakers have voted down twice. She did not say when the vote would happen. Though she did say delaying Brexit did not rule out the possibility that Britain could leave without a deal.
The Labour Party said by choosing a short delay May was forcing British lawmakers to decide between accepting a deal they have already rejected or leaving without a deal.
Many pro-Brexit members of May's Conservative Party are opposed to a longer delay because they fear it could mean Brexit might never happen. They argue Britain can do well outside the European Union even though an abrupt departure would cause short-term pain.
The loss of Britain for the EU is the biggest blow yet to more than 60 years of effort to forge European unity after two world wars, though the 27 other members of the bloc have shown surprising unity during the tortuous negotiations.
EU leaders are expected to decide on May's request for a Brexit delay at a summit in Brussels on Thursday. But some diplomats said the final decision could be pushed into next week.