Corbyn faces opposition from at least 60 MPs to a customs pact with May without a second vote

Jeremy Corbyn will not be able to get enough of his MPs to back a Brexit deal without the promise of a second referendum, even if Theresa May makes a big offer on a customs union and workers’ rights this week, senior Labour figures believe.

Senior party sources said they believe two-thirds of Labour MPs, including several shadow cabinet ministers and many more frontbenchers, would refuse to back a deal without a people’s vote attached.

Theresa May is preparing to make new proposals of a temporary customs union until the next election, matching EU employment rights in the future and alignment of single market regulations on goods.

The prime minister wrote an article in the Mail on Sunday urging Labour to “put our differences aside for a moment” to do a deal when negotiators meet on Tuesday, while Corbyn said after the local election results that voters wanted MPs to “get a deal done” on Brexit.

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John McDonnell, the shadow chancellor, had said on Friday that the election results meant voters were saying “sort it” on Brexit but he sounded a more cautious note about the prospect of a deal on Sunday, telling the BBC’s Andrew Marr Show that he had no trust in May and accusing her of negotiating in “bad faith”.

He said any customs union would need to be permanent and warned No 10 that “to get any deal over the line you’ve got to recognise there will be a large number of MPs in parliament who actually do support a public vote”.

Labour is incensed that details of May’s intended offer were leaked to Sunday newspapers when they had kept their side of a bargain to keep the talks confidential. Asked whether he trusted the prime minister over the talks, McDonnell said: “No, sorry, not after this weekend when she’s blown the confidentiality I had and I actually think she’s jeopardised the negotiations for her own personal protection.”

McDonnell also raised the issue of May’s weak position as leader and the threat of hard Brexiters vying to take over from her, likening the cross-party talks to “trying to enter a contract with a company going into administration and the people who are going to take over are not willing to fulfil that contract”.

The shadow chancellor made clear that Labour would need changes to the withdrawal agreement and political declaration negotiated with the EU to make sure future Tory leaders could not tear up the agreement after Brexit.

More than 100 opposition MPs, including 66 from Labour, said at the weekend they would not tolerate a “Westminster stitch-up” on a Brexit deal without a second referendum.

But a number of MPs close to the People’s Vote campaign believe there are actually more like 150 to 180 Labour MPs out of 229 who will refuse to back a deal struck with May unless there is a confirmatory vote.

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One shadow cabinet minister said: “Jeremy cannot be sure he has the numbers – even if he whipped it – so he cannot do a deal without a confirmatory vote.”

Several MPs talked of more colleagues quitting the party, frontbench walkouts and a “catastrophic split” if the leadership were to pursue a policy of a deal without another referendum.

May is opposed to offering a second referendum but parliament could decide to back one if the talks collapse and the government proceeds with a plan for binding votes on what MPs would prefer as an outcome. In that scenario, there could be a binary referendum on May’s Brexit deal or some kind of three-way poll on Brexit with the options of a deal, no deal or remaining in the EU.

Although striking any deal with May would be controversial within Labour, some within the party, such as party chair Ian Lavery and MPs Lucy Powell and Stephen Kinnock, have been arguing that the referendum result needs to be respected. There is also an argument being made that it could be the best way of drawing a line under Brexit and moving on to talk about other pressing national issues such as schools and hospitals.

Keir Starmer, the shadow Brexit secretary, is sticking to his promise to keep the negotiations confidential but a number of MPs said they had been reassured personally that he would not sign up to any deal without a public vote.

Even some of those who had been considering backing any cross-party deal were spooked by May’s Mail on Sunday article that tries to sell a temporary customs union until 2022 on the grounds that whoever wins a general election can set the future direction. They fear this could amount to signing up to a “blind Brexit” in which hard Brexiter successors to May such as Boris Johnson and Dominic Raab could set the terms of the future relationship.

Tom Watson, the party’s deputy leader who has been pushing for an unequivocal second referendum policy, told the BBC Radio 5 Live’s Pienaar’s Politics that “the Labour party membership and vast numbers of my colleagues in parliament don’t want us to just sign off on a Tory Brexit”. He said a deal would only be done if Starmer, who backs a second referendum, approved it.

Meanwhile, May is facing the prospect of a Tory revolt and losing votes for her deal if she makes any promise of a permanent customs union, with at least 100 of her MPs unwilling to vote for such a compromise.

Two senior Conservatives, Ruth Davidson, the Scottish Tory leader, and Rory Stewart, the international development secretary, both offered a view that the major parties needed to find a compromise. Stewart even suggested a split in the Tory party could be a price worth paying to get a Brexit deal done, telling Pienaar’s Politics: “Yes, there will be short-term pain.”

He also urged the prime minister to find a binding solution on a customs union with Labour, claiming: “We need investors to feel this will be there for 30 years not just for four years and a cross-party deal is a way to do with.”

However, the idea of a binding customs union incensed Conservative Brexiters, who are ramping up their efforts to oust May in the wake of the dire local election results for the party.

Sir Graham Brady, the chair of the 1922 Committee of backbenchers, warned May in the Telegraph that she must not cave on the issue of the customs union.

“The temptation for the government now to do whatever is necessary to secure some kind of Brexit agreement is obvious but it must be resisted,” he said.

“To reach an agreement with Labour that locked the United Kingdom into the customs union might pull in enough Labour votes to allow an agreement to limp over the line but the price could be a catastrophic split in the Conservative party and at a time when the opposition is led by dangerous extremists, the consequences for our country would be unthinkable.”

Brady is expected to lead a delegation of the 1922 to see May on Tuesday asking her to set out a timetable for her departure or face serious attempts to remove her.