Two senior Labour frontbenchers have thrown their weight behind calls for a second referendum on a cross-party Brexit deal to gain the backing of their MPs.

Shadow Brexit secretary Sir Keir Starmer warned it was “impossible” to see how an agreement between the Conservatives and his party could clear the Commons unless it guaranteed the deal would be put back to the public for a “confirmatory vote”.

And Labour’s deputy leader Tom Watson said he thought the way out of the impasse was a “confirmatory ballot” on Theresa May’s agreement, saying it would be “difficult” for his party to assist in the UK’s exit from the EU without another referendum.

Ahead of the continuation of cross-party talks on Monday afternoon, Sir Keir told the Guardian that “probably 120 if not 150” of the party’s 229 MPs could vote against the deal unless it was linked to a second referendum.

And he said: “I’ve made it clear that at this stage, at this 11th hour, any deal that comes through from this Government ought to be subject to the lock of a confirmatory vote.”

Mr Watson told BBC Radio 4’s Today programme: “If a deal could be found that inspires enough votes in Westminster then fine, but it seemed to me that that’s very, very difficult.

“And so my idea of a confirmatory ballot is not a religious point or a point of ideology, it’s just how do you get an outcome, how do you sort this out?

“And one way to do it are these two minority positions – the Prime Minister’s deal and those that think the people should have a say on the deal – plug them together and you build a majority.”

Mr Watson also described Labour’s position in relation to the European elections as a “remain and reform” party.

He is expected to plead in a speech to the Fabian Society on Monday for supporters to back Labour in the polls next week.

“There are only two forces that can win this election – that nasty nationalism of the Farage Brexit Party, or the tolerant, compassionate outward looking patriotism of the Labour Party,” Mr Watson will say.

“I can only plead with Labour supporters- don’t stay at home, don’t put that cross elsewhere, don’t let them win.”

Last month Jeremy Corbyn saw off an attempt to commit the party to a confirmatory referendum on any Brexit deal, and Labour will instead only back a fresh vote only if it cannot either win the changes it wants to Mrs May’s deal or secure a general election.

Housing Secretary James Brokenshire said a confirmatory referendum would be “taking us in a different direction – that’s actually saying ‘well, we’re looking at this issue all over again and not giving effect to the original vote”‘.

He described the cross party talks as “very serious”, adding: “We wouldn’t have committed all of the time and effort on all sides in relation to this.”

Elsewhere, the Chancellor’s parliamentary aide, Huw Merriman, said the Tories will suffer “an absolute mauling” in next week’s European Parliament elections.

“The public will blame the Conservative government because we were the party that brought forward the referendum,” Mr Merriman, who backs a second referendum, told BBC Radio 4’s Westminster Hour on Sunday night.

“And so for those that didn’t want it and wanted Remain, they’ll blame us for having tried to take us out.

“And for those that voted to leave, they’ll blame us for having not got the country out of the EU.

“We’re at the perfect storm, so yes, I think we’ll get an absolute mauling.”

Harriet Line is Press Association Political Correspondent.