Jeremy Corbyn is poised to back a plan to block a no-deal Brexit as pressure builds within Labour and the trade unions for a delay to Britain’s EU departure.

It is understood that the leader and his shadow cabinet team are preparing to support a proposal that would force Theresa May to request an extension to Britain’s EU membership should no Brexit deal be agreed by early March.

The plan would need the endorsement of the Labour frontbench to have a chance of being passed when the next round of critical votes takes place next week. While no final decision has been taken, senior figures said the move was in line with Corbyn’s demand that May take a no-deal Brexit “off the table”.

The deliberations come with the Brexit options narrowing for Labour’s leadership amid an internal battle over whether it should back a second referendum. Having tried and failed to secure an election, figures in the party say the choice is now between a Norway-style soft Brexit, which would effectively have to include free-movement rules, and another public vote.

The shadow Brexit secretary, Keir Starmer, who has been trying to keep open the option of a second vote, repeated the demand on Saturday. “A public vote has to be an option for Labour,” he told a Fabian Society conference. “After all, deeply embedded in our values are internationalism, collaboration and cooperation with our European partners.”

There are also continued efforts among Labour’s overwhelmingly pro-remain membership to push it towards backing a second referendum. In a letter in the Observer, more than 170 activists call for a special party conference on Brexit to endorse the idea. “A Norway-style deal would leave a Corbyn government with no say over laws which affect us,” they write. “We therefore call on the Labour party NEC [national executive council] to call a special one-day conference to determine Labour’s position on Brexit in light of recent events.”

Labour MPs are planning to ambush the leadership at a gathering of the parliamentary party a week on Monday. MPs want to force a debate at the meeting on the party’s position on a second referendum.

Dave Prentis of Unison has suggested that article 50 should be suspended. Photograph: Christopher Thomond/The Observer

However, several shadow ministers have told senior party figures in the last week that they would resign should the party back another referendum. “There are an awful lot of Labour MPs, even some who backed remain, who just aren’t there yet,” said a senior party source. “It is a real political problem. They might get there, but there is some way to go.”

Many figures are coalescing around a delay to article 50, the legal process that dictates that Britain will leave the EU – with or without a deal – at the end of March. Writing in the Observer, Dave Prentis, the leader of Britain’s biggest union, Unison, says the time has come for a suspension of article 50. He suggests that a so-called people’s vote should be backed only if a softer Brexit cannot be agreed.

“The chances of forcing an election anytime soon are receding,” he writes. “Other options now have to come into play ... It’s vital immediate steps are taken to extend article 50. Not to stop Brexit, but to ensure there’s time to force a change of government, or find a solution parliament can back.

“A customs union would minimise the risk of a hard border in Northern Ireland … A Norway-like deal, allowing the UK to participate in the single market too, would further minimise the risk to our fragile economy and to employment rights.”

In a sign that there is still some way to go before Labour backs a second referendum, he says such a policy should only be adopted “if other avenues fail”.

Under a plan devised by the former Tory minister Nick Boles and Corbyn’s former leadership rival Yvette Cooper, parliamentary rules would be temporarily suspended, allowing parliament to pass a law forcing May to try to delay article 50 if no deal looked likely. It would require frontbench Labour support, which is now looking likely.

With anger among some members about Brexit growing, several party sources suggested that Labour’s huge membership had begun to fall. Several party sources said that the number of paying members had fallen well below 500,000 – meaning tens of thousands had left. However, the suggestions were denied by a senior Labour source, who said: “We are proud of our mass and vibrant membership and claims about this sort of drop-off are just wrong.”

The union movement is also split on Brexit. Manuel Cortes, general secretary of the Transport Salaried Staffs’ Association, said remaining in the EU was the “best outcome for working people”.