Jeremy Corbyn will defy calls to change course on the party’s Brexit policy ahead of parliament’s vote on the deal, insisting that the government should secure a new deal with the EU if MPs reject Theresa May’s agreement.

Under increasing pressure from Labour members and MPs to reconsider his approach as preparations for the delayed “meaningful vote” ramp up over the next week, Corbyn said on Wednesday that the party’s policy remained “sequential” and that no decision could be made on a second referendum until parliament voted down the deal on offer.

His remarks come as Westminster gears up for the end of recess and the return in earnest of the Brexit debate. MPs are expected to hold the delayed vote in the second week of January.

With Corbyn’s position coming under increasing scrutiny ahead of the crucial vote, it is understood that a number of high-profile leftwing Labour figures, including Ann Pettifor, a former adviser to the shadow chancellor, John McDonnell, as well as the economics commentator Paul Mason, and Manuel Cortes, the general secretary of the TSSA trade union, are in advanced discussions about forming a policy commission to make the left’s case for remaining in the EU.

Their planned intervention follows the publication of a new study revealing that an overwhelming majority of party members want the Labour leader to back a second referendum, though most remain loyal to Corbyn’s leadership.

Corbyn and several of his closest allies have been both publicly and privately sceptical of the policy, and the Labour leader has said in a previous interview with the Guardian that the party would pursue a negotiated Brexit deal even if it won a snap general election.

Corbyn said May should return to Brussels once her deal is voted down to find an agreement that Labour could support, including a full customs union.

“What we will do is vote against having no deal, we’ll vote against Theresa May’s deal; at that point she should go back to Brussels and say, ‘This is not acceptable to Britain’ and renegotiate a customs union, form a customs union with the European Union to secure trade,” he said.

Negotiations for any future permanent customs union with the EU would be likely to form part of the next stage of talks, once the UK has agreed the terms to leave in March. MPs are due to vote on the negotiated withdrawal agreement as well as a political declaration on the future relationship, which is not legally binding.

Labour frontbencher Chi Onwurah said the party’s policy should be to push for an extension to article 50, rather than to allow May to run down the clock towards no deal, a move also backed by the TUC.

In an article for the Guardian, Onwurah, the shadow minister for industrial strategy, said the threat of no deal was “an act of supreme economic sabotage” and would not be carried out. “We must recognise the challenge of negotiating a deal that obtains cross-party support before the March deadline,” she said. “Labour’s conference motion does not reference an article 50 extension, but we have acknowledged it may be necessary.

“Increasingly, it is clear that the options to avoid an economic and social catastrophe are a general election, a public vote and/or article 50 suspension. With each day that passes, Theresa May’s inept blackmail makes it harder to do anything responsible without stopping the clock.”

After a lull over the Christmas recess, pressure on May is also expected to step up again as MPs prepare to return to Westminster.

Over the festive period, May embarked on a round of telephone diplomacy with EU leaders, speaking to the German chancellor, Angela Merkel, on Christmas Eve and again on Wednesday, as she attempted to negotiate a new legally binding wording to provide reassurances on the Irish backstop arrangements.

The Brexit secretary, Steve Barclay, is to meet junior ministers from all other government departments to receive updates on no-deal planning at midday on Thursday, the Guardian understands.

A Whitehall source described the meeting as a “stock-take” and said all departments had assigned one junior minister the responsibility for no-deal planning, after the cabinet agreed to significantly increase preparations before Christmas. No other cabinet ministers are expected to attend.

The study of Labour members found that 72% believed their leader should back a second referendum. The research, part of the Party Members Project led by Prof Tim Bale of Queen Mary University of London, found that while Labour members still strongly supported their leader overall, they appeared to be sceptical about his reasons for refusing to support a referendum.

However, the study also found a majority of Labour members said they supported the party’s current position, with 47% in favour versus 29% who opposed it.

The party’s policy, which was decided at its annual conference in Liverpool, is that there is a priority to force a general election and only after that could a second referendum be considered.

Corbyn restated that position on Wednesday. “The issue of another referendum was of course one of the options, but that was very much after the votes have taken place in parliament,” Corbyn said on Wednesday. “We haven’t yet had a vote, and I think the government really should be ashamed of itself. This vote has been delayed and delayed and delayed. This government is just trying to run down the clock and create a sense of fear between either no deal or May’s deal.”

Backbenchers urged the Labour leader to plot a path towards a second referendum. MP Phil Wilson said the leadership was “hidden behind myths” that Labour voters were evenly divided on the issue, when the study had shown they were overwhelmingly in favour of remaining.

Luciana Berger, another supporter of the People’s Vote campaign, said it was wrong-headed to believe that either Corbyn or May could negotiate a better deal. “No renegotiations are on offer and the EU have been clear that the Irish backstop is unavoidable in all circumstances,” she said. “All the evidence shows that Labour voters and Labour members are at one in their support of a People’s Vote. It’s time the leadership stopped hiding behind process and faced up to the scale of the crisis and took the side of the people.”