Boris Johnson is “a significant drag on Tory fortunes”, according to a poll that tested the Conservative party’s chances at the next election with different leaders at the helm. The current chancellor, Rishi Sunak, has a much better chance of challenging Keir Starmer’s Labour than the incumbent prime minister, according to the poll by Opinium shared exclusively with the Guardian. The research found the party would be 60 seats worse off under Johnson, compared with Sunak. However, other potential leadership rivals the foreign secretary, Liz Truss, and the levelling up secretary, Michael Gove, fared worse than Johnson, with the former estimated to lose 40 more seats and the latter 70 seats more. The pollster found 39% of people asked intended to vote Labour at the next election, seven points ahead of the Conservatives on 32%. But when Johnson’s name was added to the voting intention question, Tory support dipped below 30%, and Labour’s lead increased from seven to 12%. Sunak emerged as by far the most attractive successor, with Labour maintaining its lead but by just three points if the chancellor was to take over as leader. The nine-point difference between a 12-point and three-point lead was worth about 60 seats that Tories would lose with Johnson but retain with Sunak, the polling suggested. Commenting on the findings, Peter Kellner, the former president of the YouGov opinion polling organisation, said: “It might be expected that simply reminding voters of the names of the two main party leaders would make little difference to voting intentions. “The fact that it shifts the party lead by five points suggests that Johnson is now a significant drag on Tory fortunes.” Voters were asked: “Please imagine that at the next election [NAME] was leader of the Conservative party and Keir Starmer was the leader of the Labour party. Who do you think you would vote for?” As well as Johnson, Opinium asked people how they would vote under three further scenarios: if Sunak, Liz Truss or Michael Gove led the Tories – and in each scenario assumed that Starmer remained Labour’s leader. Truss is far less popular with the wider public than with Conservative activists, the polling suggests. The latest survey of party members conducted by the Conservative Home website showed her as their favourite successor to Johnson, with Sunak second. However, Opinium’s figures suggest that she would do even worse than Johnson, and far worse than Sunak, and lead the Tories to a crushing 16-point defeat. Gove, however, would do worse again, according to the poll. With him as leader, the Tories would lag Labour by 18 points. Kellner added: “As with all such polls, the numbers may change. Truss could reasonably argue that she is not yet as well known as Sunak; were she to be elected party leader, she would have ample opportunity to burnish her image. “In contrast, Sunak might face a difficult spring, as living standards fall, with inflation outstripping pay, and tax rises coming in April. “Even so, the gulf between Sunak and Truss is currently big enough to make the choice of Truss as leader a clear electoral risk.” The seat predictions are based on a uniform swing, that is, they take no account of local factors, tactical voting or boundary changes and should be regarded as a broad guide, not a precise projection. The research showed Johnson projected to win 203 seats, compared with 263 under Sunak, 162 under Truss and 136 if the party were led by Gove. The Conservative party currently holds 361 seats; 326 are needed to form a majority government. The poor polling for Johnson, which also includes a poll commissioned by the Sunday Times showing on Sunday Labour surging to an eight-point lead over the Tories, comes after a torrid period of sleaze scandals for Johnson and his party. The Owen Paterson lobbying scandal that ultimately led to the Tories losing a seat they had held for 200 years at a byelection, the refurbishment of the prime minister’s Downing Street flat and various claims of lockdown parties in Whitehall and No 10 have all damaged Johnson and the Conservatives’ reputation, against a backdrop of surging Covid cases and the prospect of more restrictions nearly two years into the pandemic.