Keir Starmer will step up warnings about the risks of a “blind Brexit” in Brussels on Wednesday, as Labour seeks to shift the debate from the Irish backstop to Britain’s future trading relationship in a bid to win over wavering MPs.

The shadow Brexit secretary is keen to underline Labour’s continuing objections to Theresa May’s Brexit deal, even if she secures agreement with Brussels over the Irish backstop – the key remaining sticking point in the withdrawal agreement.

He will stress the party’s concerns that the political declaration setting out the framework for the future relationship between the UK and the EU will be too vague to reassure businesses or win over sceptical MPs.

“A blind Brexit could prolong business uncertainty and provide insufficient guarantees to protect jobs, the economy and rights. Whether you voted leave or remain, nobody voted for the purgatory of permanent negotiations,” he said.

He points out that May and her Brexit secretary, Dominic Raab, have suggested that the deal will be “detailed, precise and substantive”.

“That is exactly what Labour expects and what I will be discussing in Brussels. If the final deal is anything less than the government has promised, Labour will not support it,” he said.

Brussels has long made clear that the political declaration will not be legally binding – unlike the withdrawal agreement, containing the £30bn-plus financial settlement and the backstop – and could be open to renegotiation by a future government.

Anand Menon, director of the UK in a Changing Europe thinktank, said it had long been obvious that the political declaration would not be a detailed document because of the EU’s insistence that it cannot negotiate the details of a new trading relationship with an existing member.

“A blind Brexit is inherent in Brexit. It’s inevitable,” he said. “The two negotiations will take place under a different process, with different people – if you’re anti-blind Brexit, you’re anti-Brexit.”

Starmer and the shadow chancellor, John McDonnell, have been meeting Labour MPs individually to discuss the pros and cons of the government’s likely deal and their party’s strategy when parliament votes on it.

Their hand was strengthened on Tuesday by the results of a consultation with Momentum members, which showed that they overwhelmingly want Labour MPs to vote down May’s Brexit deal. Many are hoping that they will also throw their weight behind a second referendum.

The grassroots campaign group asked its members their views on Brexit after Labour’s conference in Liverpool agreed a motion calling for the party to push for a general election and, failing that, a referendum.

Of the 6,500 members who replied to the survey, 92% said they would like to see all Labour MPs vote against the government’s Brexit deal when she brings it to parliament, likely to be in the next few weeks.

Some Labour MPs who represent Brexit-backing constituencies, including Caroline Flint and Gareth Snell, have suggested that they could support the deal, rather than risk a damaging no-deal exit from the European Union next March.

Asked about the possibility of a referendum on the final Brexit deal, just 17% of Momentum members said they would not support a vote, while 41% said they would go further than the approach of the party’s conference resolution and support a poll in any circumstances.

Another 28% said a referendum should be “an option if there is no general election” – the wording of the motion – and another 12% that Labour should commit immediately to backing a poll if they cannot secure a general election.

Labour plans to table a no-confidence vote in the government within days if May’s deal is rejected by parliament.

Becky Boumelha, a spokeswoman for Momentum, said: “Propping up a destructive Tory deal will be kryptonite to Labour members and any Labour MP who votes with the government will have a hard time explaining themselves back in their constituencies.”

Momentum was set up to back Jeremy Corbyn’s leadership, but its members’ enthusiasm for rejecting Brexit contrasts with the more cautious approach of the Labour leader and his shadow cabinet, who have repeatedly insisted that they respect the result of the 2016 referendum.

Backers of another referendum seized on the survey results. David Lammy, MP for Tottenham and member of the People’s Vote campaign, said: “It’s great news that Momentum will be campaigning for Labour MPs to oppose this miserable Brexit. Labour voters and Labour MPs should unite against a bad Brexit and work to give people the democratic voice they deserve.”

Brandon Lewis MP, the Conservative party chairman, said: “Labour have spent the last two years doing everything they possibly can to frustrate the Brexit process. They promised to respect the referendum, but voted against the EU Withdrawal Act and now want to rerun the referendum, tearing up their pledge.”