A “progressive alliance” between Labour, the Liberal Democrats and Greens would fail to stop Theresa May securing a strong majority even if it agreed to run a single candidate in every seat, a new analysis has revealed.

Some figures from all three parties have backed the idea of cooperating to increase the chances of a soft Brexit and ensure that the Conservatives do not secure a landslide. It comes as supporters of the idea prepare to gather for a major event on Monday championing the idea.

However, an analysis based on the latest polling suggests that even the most comprehensive and successful alliance between the three parties would still lead to a comfortable Conservative majority of 64 seats. There would need to be a 10% swing towards the alliance’s candidates compared with the current polling for the three parties to secure a majority between them.

The analysis will come as a blow to campaigners hoping to convince like-minded parties to work together to form a strong coalition of MPs who are willing to soften Brexit. However, supporters of the idea said that it would still help reduce May’s power and build a platform for future cooperation.

The new research was carried out for the Constitution Society by Martin Baxter, a psephologist who runs the Electoral Calculus website.

His current forecast is for a huge Conservative majority of 174 seats. Based on an unlikely scenario in which Labour, the Lib Dems and Greens make a full alliance across all seats and convince all their voters to back one candidate, the Conservatives would still have a majority of around 64 MPs.

The alliance group would require a further 6% swing from the Conservatives to deprive May of a majority. They could govern as a minority administration with the help of the SNP, should they secure an 8% swing. Even a 10% swing would only give them a majority of eight MPs.

“Assuming the polls are right, this analysis shows that even a full-scale progressive alliance across three parties, even if it were possible, could not dislodge the Conservatives this year,” Baxter said. “All the talk of small-scale local alliances with Greens or Liberal Democrats standing aside from certain seats has been shown to be peripheral, since even a large-scale alliance is insufficient.

“The right is realigning itself as Ukip fades and its voters, plus other Leave supporters, switch allegiance to the Conservatives. That change is a big driver of Tory success at the moment. The challenge for the left is both to realign itself and to get more popular. If the polls are correct, and the Labour party does indeed suffer a major defeat, then the pressure for a realignment of the left may be very vigorous after 8 June.”

Despite the findings, Neal Lawson, the chair of Compass and spokesperson for the Progressive Alliance, said limiting the size of May’s majority was important. “There is nothing we can do about the poor polling state of the Labour party,” he said. “For this to work, this requires a Labour party that is doing creditably well, and clearly we haven’t got that. That does not invalidate the principle for progressives to campaign, work and vote together, because that is going to reduce the Conservative landslide.

“This would reduce the landslide, it would prove the concept of progressive parties working together and ending up with better electoral results – and it lays the foundations for more general collaboration on policies and ideas, as well as campaigning. We never thought about this as just an instrumental, deal-making politics. For us, it is about a better political mix.”

It comes after the emergence of a number of local attempts to form a progressive alliance. However, one high-profile attempt has already failed, with party members expelled as a result of backing rival parties. Three Labour party members were thrown out recently for backing a rival candidate in a bid to unseat the health secretary, Jeremy Hunt.

Steve Williams, a Labour party member for 46 years, was told that he was now “ineligible” for continued membership because he had “publicly stated support for a party that is standing against the Labour party in the 2017 general election”.

Sir Vince Cable, the former business secretary, is the most senior Lib Dem to express support for some local pacts to defeat the Tories. He was recorded as saying he would “find it difficult” to vote against Rupa Huq, who is seeking re-election for Labour in Ealing Central and Acton.

Lib Dems in Brighton Pavilion also voted not to stand against the Green co-leader Caroline Lucas, who is defending the seat. The Greens responded by opting not to run a candidate in Brighton Kemptown.

The Lib Dem leader Tim Farron has warned that he is “very clear in our message there will be no deal, no pacts, no coalitions” at a national level.