Keir Starmer won the election because of a ruthless focus on winning over people who voted Conservative in 2019, but the party has been left with a “fragile coalition” of supporters who will abandon it if it fails to deal with the cost of living crisis and the NHS, a thinktank has said.

In a report by Labour Together, an influential Starmerite thinktank, researchers concluded voters had “cautiously hired” the prime minister “on a trial basis”, and he was “liable to prompt dismissal” if his government deviated even slightly from focusing on voters’ priorities.

The thinktank reported their findings to key No 10 figures, including Pat McFadden and Morgan McSweeney, in a Cabinet Office meeting last week.

The researchers, who are regarded as the government’s “critical friend”, said they were honest and direct about the challenges they believed lay ahead for Starmer’s top team. Officials were told that if the government did not deliver for voters, who have become more transactional than ever, they could easily face the same fate the Conservatives did after the 2019 election.

The report is based on a survey of 10,000 people across the country in polling and small focus groups, asking them why they voted the way they did. Labour Together had decided to launch this assessment of Labour’s performance once the general election was called in May, regardless of the result at the ballot box.

“Middle Britain” voters decided the outcome of the 2024 general election, the analysts said. They summarised this group of voters as being in the ideological centre of the country, slightly to the left on economic issues and more “authoritarian” on cultural issues.

The report notes it was Middle Britain’s “desire for change that mattered most” in the election, with Alex, a constituent from Leigh and Atherton, in north-west England, telling a focus group: “I’ve always voted Conservative, but it got to the point where I was thinking there’s got to be something better than this.”

However, Labour Together notes the party’s focus on demonstrating to people in “Middle Britain” that it shared their values was not without its cost, with Labour losing seats in Bristol West and Islington North to its left; Thangam Debbonaire and Praful Nargund lost to the Green party’s co-leader Carla Denyer and Jeremy Corbyn respectively. Jonathan Ashworth, the chief executive of the thinktank, also lost his Leicester South seat to the independent MP Shockat Adam.

While the research does not include analysis of independent seats, which is expected to come at a later date, it does note that many traditional Labour voters in Muslim communities backed independent candidates because of Labour’s position on the war in Gaza. It highlights a strong correlation between a decline in Labour’s vote share and the proportion of Muslims in a constituency.

“The depth of feeling about the situation in some, particularly Muslim, communities was profound,” the report states. “Its impact on voting intention was understated ahead of the election, in part because high-quality polling of ethnic minorities is rare.”

The issue cost Labour four seats, which went to independents. In the 17 seats where independent candidates finished second, Labour holds a lower average majority than it does where the Conservatives finished second.

However, the thinktank argues Labour’s biggest threat comes from the right, with the Tories being Labour’s biggest opponent, followed by Reform. Labour holds only an average majority of 14% over the Tories across all seats where they finished second. With Reform, Labour’s majorities average about 25%.

Morgan Wild, the chief policy adviser at Labour Together and co-author of the report, said: “Middle Britain needed to see that Labour had changed since 2019. It had. The 2019 Conservative voters it attracted were in the centre ground of British politics – without them, Labour simply could not have secured this decisive victory.

“Elections are won in the centre ground – a basic electoral truth Labour has a habit of forgetting. It mustn’t do so again: instead, the government must stay relentlessly focused on voters’ top priorities.”

A government figure said: “The findings reinforce that if we take our eyes off why voters elected us, we won’t be here in five years.”