Labour is preparing to offer its most strident support yet for a second referendum by voting for a plan to put Theresa May’s Brexit deal to a public vote.

The Observer understands that the proposal, drawn up by two backbenchers, will be put to parliament when May launches her third attempt to pass her Brexit deal. She is expected to do so this week, before heading to a summit with EU leaders on Thursday.

Senior Labour figures said the proposal would represent a “big moment” in the party’s position, and comes after intense talks between Jeremy Corbyn, shadow chancellor John McDonnell and shadow Brexit secretary Keir Starmer. If passed by parliament, it would put May’s deal to a “confirmatory referendum”.

The move follows the threat of further Labour resignations over Brexit. Labour MPs said more could resign should the party not issue more decisive support for a second referendum this week.

Senior government sources said it was possible that May would refuse to hold a further meaningful vote on her deal should there be no chance of success. However, the expectation is that a vote will take place on Tuesday or Wednesday.

The Labour decision to launch the referendum vote proposal has been brought forward. Senior supporters had wanted to put it to a parliamentary vote as late as possible, to allow support to build on Tory benches as the only way out of the impasse in the Commons.

However, the plan’s authors, MPs Peter Kyle and Phil Wilson, have chosen to go ahead after anger among Labour second-referendum supporters that the party abstained on a vote over the same issue last week, proposed by independent MP, Sarah Wollaston. Kyle told the Observer: “Having now spoken in depth with Jeremy, his team and John McDonnell, I’ve seen for myself how they have moved from inquisitiveness to enthusiasm. When the moment comes, I’m not anticipating any equivocation from Jeremy after such a constructive and engaging set of conversations.”

Several Labour sources said the party would officially endorse the plan. The Labour leader’s team described talks about the plan as “useful and constructive”.

Corbyn has written to MPs across the Commons this weekend to hold more meetings to find “common ground between our respective proposals and to work together to break the impasse”.

The vote will mark a significant moment for Labour, but is unlikely to pass first time. Some Labour MPs will refuse to support it and there are not enough Tories ready to offer their backing. However, senior Tories, including cabinet ministers, believe a “confirmatory referendum” offering an option to Remain will eventually emerge as the only solution should May’s deal continue to be voted down.

Discussion of the proposal came as the prime minister battled over the weekend to win support from Northern Irish DUP MPs, whose backing is key in persuading hardline Tory Brexiters to support her deal. Some Tories told the Observer they could back the deal if May agreed to step down over the summer.

While there are signs that several rebel Tory MPs are ready to now back May, a rump of “irreconcilables”, of between 20 and 30, are still vowing to vote against the plan. These rebels are being told by whips that their position will result in a longer delay and a softer Brexit at the very least, including a permanent customs union with the EU. Such a move would stop the UK signing its own trade deals.

Kyle urged MPs from across the Commons to back his plan. “We’re not labouring under the assumption we’ll get a second bite of the cherry,” he said. “I have no clear vision of what the outcome of another meaningful vote defeat would be, other than that our politics will become even less predictable, more volatile and we would lurch further to the extremes.”

“My fear is we will lurch into wildly unpredictable territory, such as a disastrous general election or Britain crashing out of the EU, with the UK seen as a basket-case country the like of which Europe has not seen for a long time.”

A group of MPs who back a softer, Norway-style Brexit are also considering tabling their own amendment.

However, some believe any such proposal should take place only if May’s deal were to be definitively defeated, when the pool of potential support from Tories would increase hugely. “We will get more Tories when her deal is well and truly dead,” said one senior figure. “We know at least 10 cabinet ministers who would back this.”

May is attempting to convince the DUP and Tory Brexiters to back her by reassuring them that the Vienna Convention’s Article 62 could be used to break free of the Northern Ireland backstop – the part of her deal causing most concern.

Geoffrey Cox, the Attorney General, is also said to be reassuring MPs about Britain’s ability to withdraw under an arbitration system.

However, a new legal opinion by four leading QCs suggests the new advice does not change the fact that Britain could be stuck in the backstop. Lord David Anderson, Jason Coppel, Sam Wordsworth and Sean Aughey write: “In our view, these propositions provide no basis for the attorney general to revise his previous assessment of the legal risk of the UK being held in the backstop against its will.”