Replacing Theresa May with Boris Johnson, or other putative leadership candidates, would not boost the Conservative party’s chances of winning the next election, a poll has suggested.

In findings that will offer some relief to the prime minister, an ICM survey for the Guardian found that voters believe the Tories would be more likely to lose the next election if May was replaced by Johnson or five other potential successors.

The only scenario deemed likely to improve Tory prospects, according to the poll, was if the party was led by an unspecified person who was “quite young and able [and] not currently in government”.

The poll’s findings are not a robust prediction of future political opinion but do offer some indication of how the Conservatives might fare under different management; May’s position would be much more precarious if the survey predicted a better showing under a specific alternative leader.

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May’s leadership has been questioned since she called a snap election in 2017 that led to the loss of the Tories’ Commons majority. Speculation about an autumn leadership challenge has intensified since she announced her Chequers Brexit plan, prompting Johnson to resign as foreign secretary on the grounds it would not constitute a proper Brexit.

This led to Johnson soaring to the top in the regular survey of Conservative party members conducted by the ConservativeHome website about who they want as next leader, overtaking the backbench Brexiter Jacob Rees-Mogg.

But, asked by ICM if the Conservatives would have a better chance of winning the next election under Johnson, only 27% of respondents agreed, while 45% disagreed, giving Johnson a net score of -18.

Jacob Rees-Mogg, who is popular with rightwing activists despite playing down his desire to mount a leadership challenge, is also seen as a vote loser, with a net score of -19. Sajid Javid, who since being promoted to home secretary is increasingly seen as a strong candidate to succeed May, also comes out as a potential liability, with a net score of -18.

Two other leading candidates – Michael Gove, the environment secretary, and Jeremy Hunt, the new foreign secretary – are seen as even worse choices. Just 7% of people believe each man would help the party’s chances, and their respective net ratings are -38 and -34.

Of the named politicians, Ruth Davidson, the Scottish Conservative leader, comes out best: 15% said the party would do better under her, while only 22% said the opposite, giving her a net score of -7. But Davidson is not an MP, and though she is popular with liberal Tories it is hard to see how she could get a Westminster seat in time for any imminent leadership contest.

A quarter of respondents said the Tories would do better under “someone quite young and able who is not currently in government” as leader. But 20% disagreed, giving this unspecified candidate a net score of +5.

According to the detailed figures – which should be treated with caution because of the small sample sizes – the position is very different among Conservative supporters, with whom Johnson is the only candidate seen as likely to increase the party’s chances. Some 41% of Conservative-supporting respondents believe he would help the party, against 33% who disagree, giving him a net score of +8.

Rees-Mogg is seen by Tories as the person next best placed to help the Tories, but his net score is -8.

However, in a leadership contest Johnson would be challenged to show that he could win over Labour voters. The detailed ICM figures show that amongLabour supporters Johnson is seen as the most toxic of the possible successors to May, with just 14% of them saying he would help his party win an election, and 60% saying he would hinder the Tories.

The poll also shows the Tories on 40% (up one point from two weeks ago), Labour also on 40%, the Liberal Democrats on 8% and Ukip on 6% (all unchanged).

ICM Unlimited interviewed a representative online sample of 2,021 adults aged 18 or over, between 17 and 19 August 2018. Interviews were conducted across the country and the results have been weighted to the profile of all adults. ICM is a member of the British Polling Council and abides by its rules.

