Labour may not be able to repeat the conditions that led to the party’s shock election result in June, a leading “soft left” thinktank has said, suggesting in a report that a “one more heave” strategy may be fundamentally flawed.

The report by Compass, a leftwing pressure group formed by politicians unhappy with Labour’s direction under Tony Blair, will be seen as a challenge to the prevailing thinking among Jeremy Corbyn supporters that the party is assured of victory in the next general election.

It says the party’s voters are a fragile coalition, with seats at risk of being lost as well as won, especially when the party is forced to take a more concrete stance on an EU exit deal.

The report, by Matthew Sowemimo, says Labour has benefited from low expectations and unprecedented voter mobilisation, winning votes from many remainers. However, Sowemimo says Labour could face difficulties winning new seats as it becomes “the party of cosmopolitan cities” and working-class support declines.

“The decline of class-based voting and the unprecedented volatility shown during the course of the 2017 campaign indicates that the party leadership can take nothing for granted as it prepares for the next general election,” the report says. “Furthermore, it gained pro- and anti-Brexit voters – that trick may not be repeatable whenever the next election is called.”

Most of Labour’s seat gains came in places with an educated, metropolitan population, including Brighton Kemptown and Bristol North West, as well as seats with high student populations, such as Canterbury, Plymouth Devonport, Reading East and Sheffield Hallam. There were also gains in seats with high ethnic-minority populations, including Bedford.

Two seats in Southampton are a telling example, the report says: the party’s share of the vote rose by 17 points in Southampton Test, home to large numbers of students and ethnic minorities, but Labour lost in Southampton Itchen, a predominantly white working-class constituency.

In Southampton Test, students helped Labour win the seat. Photograph: Photofusion Picture Library/Alamy

“Labour has experienced a further decline in working-class voters and is still performing poorly in towns, rather than cities,” the report says. “So Labour lost Copeland again, while winning Canterbury. Labour failed to win the general election because of the loss of these working-class voters, particularly in the Midlands and the north-west.”

The report says there are few votes left for Labour to claim among non-voters or leftwing voters. It says 60% of people who did not vote in the 2015 general election voted Labour in 2017.

“Voter volatility is now high: 20%, or over 6,500,000, voted tactically on 8 June 2017, and party identification is at an all-time low. People move to and from parties with much greater ease,” the report says.

“One risk is that Labour’s leadership acts as if those in the centre have nowhere to go and takes their support for granted, just as the Blair leadership took the left for granted – politics abhors vacuums, as we have seen. A rejuvenated Liberal Democrat party or a new party could target this ground successfully.”

In recent years, the think-tank has lobbied for the adoption of a progressive alliance tactic, which would see Labour, Lib Dem and Green candidates stand aside in seats to allow the party with the best chance of defeating the Conservatives a clear run. The report found that in more than 60 seats, “the wasted progressive votes were bigger than the margin of victory for the Tories.”

Senior Labour politicians, including deputy leader Tom Watson, have expressed scepticism about the tactic, arguing that for Labour to be seen as a party of government, it must stand candidates in every seat.

Sowemimo argued Labour would need the Lib Dems to defeat the Tories in a number of seats where they were the main challengers in order to deliver a Corbyn victory. “Labour’s support in the 2017 general election hit against clear demographic limits,” he wrote. “This underscores the importance of the party leadership embracing wider forms of political mobilisation that can channel progressive voters effectively.”