A deal to ensure a smooth British exit from the European Union is still possible but the risk of a no-deal is also "very real," European Commission President Jean-Claude Juncker said Wednesday.

In an address in the European Parliament, Juncker said British Prime Minister Boris Johnson had told him on Monday that London still wanted a transition deal, but that he would leave with or without an accord on Oct. 31.

Pro-Brexit lawmakers cheered and applauded in the Strasbourg chamber.

"There is very little time left...The risk of a no-deal is very real," said Juncker, who met Johnson in Luxembourg on Monday.

Juncker repeated his public message, saying London must present realistic proposals to replace the Irish backstop arrangement in their Britain-EU divorce agreement.

"I am not emotionally attached to the Irish backstop," Juncker said. "I have asked the prime minister to make, in writing, alternatives," he said, calling it a safety net to avoid a divided Ireland after Brexit.

His pessimistic tone was echoed by Finland's minister for European affairs, Tytti Tuppurainen, who also spoke in the parliament, saying a no-deal Brexit "is a quite likely outcome." Finland currently holds the EU's rotating presidency.

However, many EU lawmakers warned against a no-deal. The parliament is due to adopt a resolution later on Wednesday calling for Britain to be granted another extension to allow more time for London to agree the terms of its withdrawal.

The EU's chief Brexit negotiator Michel Barnier on Wednesday said the European Union was in no mood to pretend to negotiate, levelling a warning at London's lack of concrete proposals.

"It is certainly not a question of pretending to negotiate," Barnier warned in the European Parliament.

In "this extraordinary and complex negotiation... it is our responsibility to pursue this process with determination, with sincerity," he told MEPs.

"I advise everyone not to underestimate the consequences, clearly for the United Kingdom first of all but also for us, of the absence of a deal," Barnier said.

Spelling out the need for the backstop, Barnier warned that even if Britain leaves without any agreement at all, major problems will still have to be resolved, including the future of citizens hit by Brexit, peace in Northern Ireland and the protection of the EU's single market and the Irish economy.

"None of these questions disappears," Barnier said Wednesday, insisting that the challenges must not be underestimated. "We need legally operative solutions in the withdrawal agreement to respond precisely to each problem — to address each risk — that Brexit creates."