The Conservatives are on course to lose more than 800 council seats in next month’s local elections in results that, if repeated at the next general election, would see Sir Keir Starmer, the Labour leader, become prime minister.

Pollsters Electoral Calculus and Find Out Now are forecasting a five per cent swing from the Tories to Labour at the local elections in England and Wales on May 5.

If replicated at a general election, the figures suggest Labour would emerge as the largest party in Parliament, 15 MPs short of an overall majority and probably reliant on a power-sharing deal with the SNP to form a government.

The polling firms asked about the voting intentions of more than 12,000 people in 201 district and unitary councils between April 4 and April 8. The sample was then weighted by gender, age, social class and past voting pattern.

Electoral Calculus and Find Out Now found the Tories are likely to lose 810 seats – with their wards falling from 1,965 to 1,155 – while Labour will gain 835, giving the party 3,722 predicted wards.

Prized Tory councils including Wandsworth – a totemic authority for the party as an early adopter of Thatcherite policies of council house sales and privatisation of street cleaning and refuse collection – are forecast to be taken by Labour.