Labour has established a clear lead over the Tories in more than 100 battleground seats that will decide the next election, according to a new analysis seen by the Observer. In a rare insight into marginal constituencies at next year’s election, Keir Starmer’s party has now secured a 10% swing from the Conservatives in a set of 144 seats in which the vote will be won and lost. Labour now holds a seven-point lead over the Tories in the seats, which include marginals in England, Wales and Scotland. With both main parties increasingly refining their key policy offers ahead of a looming election, the research suggests Labour now sits on 39% of the vote in the marginal seats, up from 32% at the last election. Meanwhile, the Tories have slumped from 44% to 32%. However, the polling suggests that Labour’s hopes of securing an overall majority remain on a knife edge. To achieve this, Labour needs to win an additional 124 parliamentary constituencies. The analysis suggests it is on course to win about 117 seats from the Conservatives in England & Wales and about six to eight from the SNP in Scotland. That would give that party 123-125 extra seats. Worryingly for Rishi Sunak, Labour has also amassed a huge lead among the under-50s in the seats, which include many of the “red wall” constituencies that the Tories won for the first time in 2019. Labour leads by 28 points among those under 50. The rare analysis examining the battleground seats features in a report for the UK 2040 Options project, overseen by the Nesta thinktank and the Behavioural Insights unit, which aims to set out the policy options for tackling the biggest challenges facing the country over the next two decades. Opinium polled 4,000 voters living in key marginal constituencies. It picked the 124 constituencies that Labour came closest to winning and combined them with the 100 constituencies most narrowly won by the Tories. Many seats were in both groups, giving them 144 battleground seats. Both parties are now planning their strategies for the next election and attempting to bolster their reputation for fiscal credibility. Labour is blaming the Tories for the increase in mortgage costs hitting millions of homeowners and the Conservatives are warning that Labour’s green investment plans would push up inflation. The marginal seats research found that the economy and the NHS are the overwhelming priorities for voters there and are likely to dominate the election. Nearly a third (31%) of voters think inflation and cost of living is the most urgent issue, followed by NHS staffing and funding (20%). One in 10 voters selected immigration as the most important issue to tackle, although this rose to 19% among the over-65s. Tory MPs expect tax cuts to be made next year, with most hoping for a cut to income tax as the headline election offer. However, the research suggests that tax cuts are not a priority for voters in the key set of seats. Just 12% expressed a preference for cutting taxes and spending less on public services, with 38% wanting to keep tax and spending as they are. A further 36% want to raise taxes and spend more on public services. Labour appears to be making the most significant progress in England in the Midlands, where it lost significant ground to the Tories at the last election. It has secured a 12.7% swing from the Tories in the region. Ravi Gurumurthy, Nesta’s chief executive, said that the analysis made it abundantly clear that the state of the economy and the NHS were the dominant issues ahead of an election next year. “The public is often represented as being polarised and divided, but there’s a lot of agreement among voters about the issues we need to solve right now and for the country’s future,” he said. “The public primarily wants the government and politicians to focus on the economy and health services. We’ve had quite a few handbrake turns on policy. Regardless of who forms the next government the UK is in desperate need of an extended period of stable policy-making. This means levelling with voters about trade-offs. People deserve grown-up debates about the decisions that will affect their lives now and in the coming decades.”