The government is pushing ahead with changes to electoral law that could make it easier for Conservatives to win future mayoral elections.
Home secetary Priti Patel has already unveiled plans to introduce the first past the post electoral system used to elect MPs in Westminster.
Mayors across England are elected from the existing supplementary vote system, in which the public ranks their two favourite candidates.
Prof Tony Travers, of the London School of Economics, said analysis of Thursday’s polls suggested this change could open a potential route to victory for the Tories in cities such as London.
“It’s likely that first past the post would make it somewhat easier for the Conservatives to win if they could come up with a really good candidate,” he said.
Labour’s Sadiq Khan won the London mayoral contest comfortably against his Conservative rival, Shaun Bailey, once voters’ second preferences were taken into account. But Khan beat Bailey by only 40% to 35% on first preference votes, as some leftwing former Labour voters shifted to the Greens and other smaller parties.
Although Labour won most mayoral elections across England easily – with Andy Burnham receiving a landslide 67% of votes in Greater Manchester – under the new voting system it would be likely to lose others such as the Cambridgeshire and Peterborough mayoralty.
There is also evidence that voters are confused by the use of the supplementary voting system. Some 5% of ballots cast in this year’s mayor of London election were rejected because people had voted for too many candidates.
The government will have to pass fresh legislation to change the voting system, which would also affect elections for police and crime commissioners.
Labour has pledged to oppose the changes.