British Prime Minister Theresa May on Saturday defended her decision to turn to Britain's main opposition to get her EU divorce deal approved, warning without cross-party consensus Brexit could "slip through our fingers."

The beleaguered leader opened talks this week with the Labour Party in a bid to break months of stubborn opposition in parliament to the withdrawal agreement she struck with European leaders last year.

MPs have rejected three times her deal finalized with the bloc last November to end 46 years of membership.

May's overtures to Labour came ahead of an EU summit on Wednesday where she must secure another Brexit extension, until Jun 30, to prevent Britain crashing out the bloc at the end of next week with no accord.

The country's original March 29 departure date was delayed to April 12 last month. 

"We must deliver Brexit and to do so we must agree a deal," May said in a statement released by Downing Street, adding the two main parties agreed on major aspects of Brexit. 

"That is the basis for a compromise that can win a majority in parliament and winning that majority is the only way to deliver Brexit." 

"The longer this takes, the greater the risk of the UK never leaving at all. It would mean letting the Brexit the British people voted for slip through our fingers."

However, after several days of negotiations with Labour, its leader Jeremy Corbyn complained he had not "noticed any great change in the government's position so far."

"I'm waiting to see the red lines move," he added.

Meanwhile, EU members, who must give unanimous backing to any further Brexit delay, are growing increasingly impatient at the dysfunction in Westminster.

They could offer just a shorter postponement - or a longer period of up to a year.

French Foreign Minister Jean-Yves Le Drian told reporters at a G7 meeting in France on Saturday that it was time for the Brexit crisis to end.

"The British authorities and the British parliament need to understand that (the EU) is not going to be able to constantly exhaust itself with the ups and downs of domestic British politics," he said.

However Irish Prime Minister Leo Varadkar cautioned it was "extremely unlikely" a member would veto another extension while revealing he now favored a lengthy delay.

(Cover image: British Prime Minister Theresa May gives a statement inside 10 Downing Street in London on April 2, 2019 after chairing a day-long meeting of the cabinet.)