Progressive voters have been driven away from Labour by a lack of argument and vision from Keir Starmer, according to a report using research from a senior pollster to Tony Blair and Bill Clinton.

Downing Street is understood to have been briefed on the research, which has also been handed to allies of the potential leadership candidates Andy Burnham, Wes Streeting and Angela Rayner.

Labour is braced for dismal results in Thursday’s elections, which could result in Starmer facing a leadership challenge.

The report from UCL’s Policy Lab, using research from the eminent pollster Stan Greenberg, suggested voters felt that Starmer had a “discomfort” with progressive values. Key fights that the government could pick included a more robust challenge to Donald Trump and a more passionate defence of environmentalism.

Greenberg’s findings suggested the next election would be won by those who can unite left and right “blocs” of voters. The polling suggested that would be achieved not only through delivery or policies, but through a clear articulation of core values.

Marc Stears, the director of the Policy Lab, said: “Keir Starmer came in on a wave of frustration with politics as usual and a promise of change. There is no hiding from the frustration voters feel with his time so far.

“Some of this can be put down to his personal style but this report suggests it is also due to his discomfort with progressive values. The party and politics that is able to tap more effectively into the core values of their potential supporters might do better in this newly polarised age.”

Examples given by the report’s authors include Starmer’s decision not to join the Iran war – which was welcomed by voters but which they said needed to be accompanied by a strong values statement about why the war was wrong, rather than technocratic explanations about illegality.

Policy Lab’s James Baggaley said there was widespread expectation that Starmer intended to use closer ties with the EU as a way of signalling a progressive turn. But he said that again would not be enough without a more explicit declaration of values. “Our research shows that progressive voters do want closer ties to the EU, but they’re also looking for much more radicalism on public services, taxation, climate and democracy,” he said.

“Unlike previous Labour prime ministers, Keir Starmer faces a serious challenge from a party to his left. How Labour responds to the Greens will shape politics in the years ahead. Indeed, what the polling shows is that progressives currently have the kind of hunger for radicalism more frequently associated with challengers from the Brexit-facing right.”

The polling found Starmer was viewed negatively by three-quarters of voters – similar levels to the former Labour leader Jeremy Corbyn. Burnham, the mayor of Greater Manchester, had the best ratings, viewed positively by a third of voters.

The polling found about 13% to 15% of progressive voters were open to voting Labour, compared with only 2% of Reform voters. Liberal Democrat voters were more likely to consider voting for Labour than the Greens. The polling also suggested Reform UK was reaching its ceiling, with few Conservative voters willing to say they would still consider voting for Nigel Farage’s party.

The polling found willingness to confront Trump brought an intensely positive dividend from voters. But the report suggested that so far most progressive voters felt Starmer had failed to do sufficiently, despite his criticism of the US president in recent months. The polling found it was a position popular with Labour voters and those who had left the party for other progressive parties – including the Greens and Lib Dems, where it was backed by more than two-thirds of voters.

Progressive voters in the report’s focus groups said Britain and the government had failed to show values-based opposition to Trump, in the way that other politicians in Canada and Spain had managed.

Almost as popular was strong messaging on the climate – a fight that Starmer has been more willing to pick in recent months to underline why energy independence is so important to bills amid the Iran war’s effect on fossil fuel prices.

Voters who responded to the report also cited a deep personal animosity to the prime minister that went beyond his ability to communicate. Greenberg and Steers described a “failure to come to terms with our newly polarised age”.