In a decision unpalatable to most British lawmakers, European Council President Donald Tusk says a delay to Brexit is possible, but only if the UK Parliament approves Theresa May’s twice defeated deal.

Mr Tusk’s comments Thursday morning (Australian time) came after the British Prime Minister wrote to the EU, formally requesting a delay in Britain’s departure from the EU until June 30.

Britain is set to leave the bloc on March 29, and if no delay is approved by the 29 EU member countries, it will face a possible default no-deal Brexit.

“I believe that a short extension will be possible, but it will be conditional on a positive vote on the withdrawal agreement in the House of Commons,” Mr Tusk said.

However, he questioned the length of the extension, noting that Ms May’s proposed date of June 30 has its merits but also “creates a series of questions of a legal and political nature”.

If parliament approves the Brexit divorce package next week, EU leaders should be able to finalise their extension decision without holding a special summit, Mr Tusk noted, while adding that he “will not hesitate” to invite them back to Brussels if necessary.

The British parliament is holding an emergency debate on Ms May’s request for a three-month delay to Brexit after an application by the opposition Labour Party.

Almost three years after Britain voted to leave the EU, and nine days before the formal exit deadline, 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 Ms May set the March 29 exit date two years ago by serving the formal Article 50 divorce papers, she declared there would be “no turning back”, but Parliament’s refusal to ratify the withdrawal deal she agreed with the EU has thrust her government into crisis.

“As prime minister I am not prepared to delay Brexit any further than the 30th of June,” Ms May told a rowdy session of parliament on Thursday morning.

“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.

Ms May said she planned to ask parliament to vote a third time on her departure deal. She didn’t say when the vote would happen.

The opposition Labour Party said that by choosing a short delay, Ms May was forcing British MPs to decide between accepting a deal they have already rejected twice or crashing out of the EU without a deal.

Pro-Brexit members of Ms May’s Conservative Party are opposed to a longer delay because they fear this could mean that Brexit might never happen.

EU leaders are expected to discuss Ms May’s request for a Brexit delay at a summit in Brussels on Thursday and Friday.

European Commission head Jean-Claude Juncker said the EU had done much to accommodate Britain and can go no further.

“There will be no renegotiations, no new negotiations, no additional guarantees in addition to those already given,” Mr Juncker told Germany’s Deutschlandfunk radio.

“We have intensively moved towards Britain, there can be no more,” he said.

-with AAP