Labour forced by May to back second Brexit referendum: Finance spokesman

"We have been forced into this by Theresa May delaying, running down the clock," McDonnell told Sky News on Sunday.

"If we can't get that we will have to break the log jam by going back to the people. It is not what we want but it is what we have been forced into," he added.

This week the party said it would back a second referendum in a bid to prevent either a “no deal” or May's deal.

May has less than a month to get parliament’s approval for her deal before the UK leaves the European Union.

Meanwhile, as many as 70 of Britain's opposition Labour lawmakers say that they oppose holding a second Brexit vote, according to Labour lawmaker Caroline Flint.

"I think there are something like 60-70 Labour members of Parliament who feel as strongly as I do against a second referendum," said Flint, who represents an area of the UK which voted to leave the EU at the 2016 referendum.

On Wednesday, British lawmakers granted May a two-week reprieve, postponing their threatened rebellion aimed at blocking a no-deal Brexit after she had agreed to a possible delay to Britain's departure from the bloc.

They approved a three-step plan that would give the premier more time to renegotiate her deal with the EU.

If May's bill fails, parliamentarians will vote on March 13 and 14 to decide whether to leave the EU without any deal on March 29 or postpone it to a later date.

After months of saying that Britain must leave the EU on time on March 29, May opened up the possibility on Tuesday of a limited extension to the exit date.