Theresa May has sparked hopes among senior Labour figures that she is laying the groundwork to compromise on her Brexit deal after next week’s meaningful vote, following a flurry of contacts with union leaders and MPs in recent days.

Conservative ministers were calling around Labour MPs on Thursday – including at least one member of the shadow cabinet – to gauge support for May’s deal if she beefs up pledges on workers’ rights.

May spoke directly to Len McCluskey, the Unite general secretary who is a close ally of Jeremy Corbyn, and Tim Roache, the GMB general secretary.

The calls – the first time she has spoken to either of the union leaders – were one demand made by a group of Labour MPs, including Gareth Snell and Caroline Flint, who met her in Downing Street earlier this week.

The MPs pushed for fresh assurances on workers’ rights. One of those present said they were then given advice from officials to help them draw up an amendment to the meaningful vote motion – which the government has since said it would consider accepting.

It is understood May opened her conversations with union leaders proffering the amendment as her “opening gambit”. No 10 described the calls as “constructive”.

Allies of McCluskey said he would be keen to keep channels open with Downing Street; but there were raised eyebrows at the timing of the call and an assumption that the prime minister was using it to send a signal to the Labour leadership.

Roache said: “I represent 620,000 working people and it’s about time their voices were heard. After nearly three years I’m glad the prime minister finally picked up the phone. As you would expect, I was very clear about GMB’s position – the deal on the table isn’t good enough and non-binding assurances on workers’ rights won’t cut it.

“If the deal genuinely did the job for GMB members, our union would support it, but it doesn’t. It’s clear more time is required, we need to extend article 50 and ultimately give the final say on Brexit to the public.”

In his September conference speech, Corbyn offered to support the prime minister’s deal if she toughened up workers’ rights and environmental standards and offered a permanent customs union.

She has flatly rejected the idea of a customs union, and did so again on Wednesday at a public meeting with a cross-party group of dozens of MPs.

But Downing Street has warned Brexiters who plan to reject May’s deal on Tuesday that there is probably a majority for a customs union in the House of Commons.

Keir Starmer, the shadow Brexit secretary, has said he believes parliament must decide the way forward if May’s deal is rejected on Tuesday.

Starmer reacted angrily on Thursday, after Downing Street suggested there would be just 90 minutes of debate on whatever Plan B May brings back after next week’s vote.

The prime minister’s spokesman told reporters at Thursday morning’s lobby briefing that No 10’s understanding of the Dominic Grieve amendment, which requires May to outline a plan B in three working days if she is defeated, was that only a limited debate would then be allowed.

The spokesman said: “[In relation to] the motion that would follow from the Grieve amendment, there would only be 90 minutes of debate on the motion, is our understanding, and only one amendment could be selected.”

MPs were quick to voice concern, with Yvette Cooper raising a point of order a few hours later in the Commons, asking the speaker, John Bercow, to clarify that the government could allow more time for any plan B Brexit debate if it so wished.

Unusually, Julian Smith, the chief whip, intervened, to insist that no final decision had been made by the government on how to handle the plan B debate – and even claimed, despite the earlier on-the-record briefing by May’s spokesman, that “the information is not correct”.

Starmer accused the prime minister of “seeking to shut down debate” and said Number 10 was “simply wrong”.

The shadow cabinet discussed how they will approach next week’s historic vote and its aftermath at their meeting this week, but no firm decisions were taken – for example, about whether to back any amendment calling for a second referendum.

“We need to wait and see what May comes back with,” said one shadow cabinet minister, adding that it was not inevitable a “crunch-point” on a second referendum would ever come, if the prime minister is willing to compromise.

The Labour party chair, Ian Lavery, made a passionate intervention at the meeting, rejecting the idea of a fresh poll, two people present said. Lavery, shadow justice secretary Richard Burgon and Corbyn’s close ally Jon Trickett are the strongest opponents of supporting a second vote, fearing that it would alienate Leave voters.

But the overwhelming majority of Labour’s members favour a referendum, putting the leadership under intense pressure to support one.

Corbyn stepped up his call for his preferred solution of a general election to “break the deadlock” at Westminster, in a speech in Wakefield on Thursday, and conceded that Labour might support extending article 50, to allow it to negotiate its own Brexit deal.

“Any political leader who wants to bring the country together cannot wish away the votes of 17 million who wanted to leave, any more than they can ignore the concerns of the 16 million who voted to remain,” he said.

He said Labour’s own Brexit plan, which included negotiating a customs deal and what the party has described as a “strong single market relationship”, would bring both sides together and allow Labour to enact its manifesto promises.

“The alternative plan that Labour has set out for a sensible Brexit deal that could win broad support is designed to enable us to fulfil those ambitions while respecting the democratic result of the referendum,” Corbyn said.

MPs will continue discussing the Brexit deal on Friday, with the home secretary, Sajid Javid, opening the third day of debate. There will be two more days of debate next week, culminating in the vote on Tuesday evening.