Jeremy Corbyn is set to pull the plug on cross-party Brexit talks unless Theresa May offers real changes to her ‘divorce’ deal plans this week, his senior allies have warned.

More than a month since the prime minister first invited the Labour leader to help break the parliamentary deadlock, the party’s patience is running thin at the lack of substantive proposals from the government.

Although some progress has been made in specific, limited areas such as workers’ rights and the environment, on the bigger issue of customs and on ‘Boris-proofing’ any deal, ministers have failed to produce plans that Labour can work with.

“These talks are not an indefinite process. We are looking to nail down in the next few days whether and how the government is prepared to move from its failed deal,” Corbyn’s spokesman said.

Both sides met for yet more talks on Wednesday evening, but Whitehall sources also said that if the latest round of talks failed to achieve serious progress then it would be time for a new approach.

Aides had warned this week’s negotiations were the ‘moment of truth’ to discover whether they moved on to a more intensive stage or were abandoned.

In another sign that the negotiations were in trouble, No.10 stressed that the PM’s ‘Plan B’ was to move away from a Labour-Tory joint deal and instead table a series of new votes in the Commons on alternative options.

The PM’s official spokesman said: “If it becomes clear that we aren’t going to be able to resolve this issue through the talks between the two parties and presenting a single position to parliament, then the ‘Plan B’ is for a series of votes to take place.”

However, HuffPost UK understands that Labour is highly sceptical of any plan to be bound by ‘indicative votes’, believing them to be another form of ‘blind Brexit’ that ties the hands of a future Corbyn government.

Under pressure from her own MPs to try to get her plans through without Labour’s official support, May is now expected to table her long-awaited withdrawal agreement bill in the next fortnight in a bid to show some momentum.

In the latest talks, held in the Commons, May was represented by her de facto deputy David Lidington and chief of staff Gavin Barwell, joined by chancellor Philip Hammond and environment secretary Michael Gove.

Labour’s team was led by shadow Brexit secretary Keir Starmer, shadow chancellor John McDonnell, shadow business secretary Rebecca Long-Bailey and shadow environment secretary Sue Hayman.

Labour wants to explore if there is a “stable majority for a sensible alternative” to May’s plan, but Corbyn’s spokesman said “we have yet to see substantive movement”.

Following a two-hour session on Wednesday, both sides admitted they had failed again to ‘nail down’ any progress but agreed to keep meeting.

“After the second day of talks this week, the negotiating teams are working to establish scope for agreement, and will meet again at the beginning of next week,” a Labour spokesman said.

A No.10 spokesman appeared slightly more upbeat. “This is the second extended meeting between the government and opposition, which demonstrates the seriousness with which both sides are approaching these talks.

“Over the coming days there will be more meetings of the bilateral working groups and further exchanges of documents as we seek to nail down the details of what has been discussed.”

Insiders say that government ministers are still trying to repackage May’s deal without offering any real shift in position.

Key sticking points have been May’s reluctance to sign up to a permanent customs union with the EU, as well as the lack of any mechanism to ensure any deal could not be unpicked by a future Tory leader.

May has also refused to countenance a confirmatory ballot as a condition of any deal, an issue that has been repeatedly raised by Labour every time talks were held.

A No.10 spokesman said: “We have always said that we are approaching these talks in a serious and constructive way. If we are going to make progress it’s going to require compromise on both sides.”

May’s deal has been rejected three times by MPs, by large margins, as Tory Brexiteers and Labour refused to sign up to what they felt were flawed proposals.

Tory MPs fear that May is using the talks to buy time for her premiership, and are putting pressure on her to clarify her departure from No.10.  She has already said she will quit only once the first stage of Brexit is delivered.

Downing Street confirmed on Tuesday that the European elections would go ahead on May 23, and suggested that ‘Brexit Day’ may now be as late as August 1 if the Commons ratifies a deal before it rises for its summer recess.