Pro-leave MPs tell May she risks a breakaway if she makes concessions on exit terms

Downing Street has ruled out any movement on customs union membership after Brexit-supporting Conservatives told Theresa May that a change of course to gain support for her deal would risk a serious party split, and possible breakaway.

The government set out its position on Wednesday night before the prime minister began Brexit talks with party leaders.

May responded to the historic defeat of her Brexit bill on Tuesday by pledging to speak to “senior parliamentarians” to identify a deal that could secure a majority vote.

However, after one leave-supporting backbencher said that if she made any concessions on the customs union “there’s a real risk to the party staying together”, No 10 stressed its commitment to the principle of an “independent trade deal”, which May does not believe is compatible with a customs union.

Those who support a permanent customs union include the Labour frontbench under Jeremy Corbyn and Tory backers of a Norway-style deal. It is unlikely talks with either group will get off the ground if May maintains her red lines, although Labour, the Scottish National party and the Liberal Democrats have insisted May must keep all options on the table before talks can begin.

Corbyn has said he will not speak to May until she rules out a no-deal Brexit.

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Privately, some ministers and MPs believe the prime minister can only achieve some form of cross-party consensus by pledging permanent customs union membership. “There is no other way,” one frontbencher said. “At some point, that penny will drop.”

No 10 has not yet spelled out how May intends to identify the kind of deal that would have a chance of winning over enough of the 128 Conservative and Democratic Unionist MPs who voted against her on Tuesday night.

The prime minister is expected to approach senior members of the European Research Group, the hard Brexit group chaired by Jacob Rees-Mogg which includes Steve Baker and Iain Duncan Smith. MPs in the hard Brexit faction suggested the deal could still be salvaged by agreeing on a hard end-date for the Irish border backstop.

Several Eurosceptic rebels warned the prime minister against any efforts to pursue a softer Brexit.

Simon Clarke, the MP for Middlesbrough South and East Cleveland, a marginal leave seat, said he feared a split if an eventual May deal was carried by Labour votes. “The PM is within her rights to try to find a sensible way forward, but there must be red lines,” he said. “Were we to adopt the terrible Labour idea of being non-voting members of the customs union, for example, I fear there’s a real risk to the party staying together.”

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Ben Bradley, who resigned his post as Tory vice-chair to vote against the deal, said May could still win if she pivoted towards the Brexiters.

“Only one single thing needs to change,” he said. “An alternative to, or exit mechanism from, the backstop would in my view get a majority in the Commons, and the longer-term discussion about customs unions or free trade agreements then becomes a matter for future negotiations.

“The DUP have consistently said that’s what they need, and I think most Brexiteer colleagues would support [that] along with leave-minded Labour MPs. The alternative – reaching out to Corbyn on a customs union deal – would split the government benches in half and make an election far more likely.”

May is expected to extend her invitation for talks to Labour and other opposition backbenchers, as well as Tory Eurosceptics.

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One cabinet source expressed pessimism that trying to achieve a softer Brexit would be effective. “Getting a customs union would be extremely difficult. It would only work if the calculation was that Labour votes were absolutely the only way of preventing no deal,” he said.

The justice secretary, David Gauke, said the government needed to show some flexibility but there were disadvantages to remaining in a customs union.

Gauke told the BBC: “I don’t think we can today be boxing ourselves in. What we need to be doing is engaging across parliament, seeing what ideas emerge, where the support is for those particular ideas, and at that point we need to make an assessment: is this something negotiable with the European Union and something with majority support in the House of Commons?

“I think the right answer would be to leave the customs union, but given where we are we have to be open to proposals that are put forward and make an assessment on the way forward.”

Gauke did not rule out holding talks with Corbyn but suggested they would be futile. “I think there may be others who might be easier to work with.”

May said in the Commons that the exercise of reaching out was about “wanting to understand the views of parliamentarians so that we can identify what could command the support of this House”.

She said engagement had to align with what she defined as respecting the vote to leave, citing “ending free movement, a fairer deal for farmers and fishermen, opening up new opportunities to trade with the rest of the world and keeping good ties with our neighbours in Europe”.

Ken Clarke, the pro-remain Tory MP, urged May to consider a customs union. “I have had to accept the majority in the House is committed to the UK leaving the European Union. She must also accept that she must now modify her red lines that she set for herself at Lancaster House and find a cross-party majority which will be along the lines I have indicated,” he said.