David Cameron’s Tories need to gain at least 250 seats in this week’s council contests to provide clear evidence that they are on course to win the next general election.

Labour, by contrast, could lose half of the nearly 500 seats and all four councils it is defending.

With the three largest parties expecting a backlash from angry voters over the Westminster expenses scandal at the European polls, it is the local elections that are likely to provide a better indication of underlying party allegiances.

The latest Sunday Times survey of local government by-election results — based on more than 50 contests across the country during the past six months — gives the Conservatives a national equivalent vote share of 40%, up nine points from four years ago. Labour is down nine points on 25%. The Liberal Democrats remain stable, but their 27% share now puts them in second place.

If those figures are reflected in the votes cast on Thursday, the Tories would gain more than 300 seats with Labour down by nearly 250. The Lib Dems could also lose out if voters switch straight from Labour to Tory candidates.

The 40% marker has important symbolic significance for the Tories.

If they do not achieve it this year, their opponents will be able to claim they have fallen back compared with the local elections in 2007 and 2008. Equally, it is recognised as the minimum share of the vote required to guarantee even a narrow House of Commons majority.

The Tories need to win seats from Labour and the Lib Dems at the general election, and the local contests give them an ideal dummy run in some pivotal areas.

In Nottinghamshire, for example, a swing of 3% from Labour to the Tories since 2005 will deprive Labour of overall control; one of 7% will give the Tories a majority.

The county also contains no fewer than five Labour-held parliamentary marginals, so the results in places such as Amber Valley, Broxtowe and Gedling will be under particular scrutiny.

A similar situation applies in the three other counties that Labour is defending on Thursday. Derbyshire, Lancashire and Staffordshire all fall on a swing of 5% or less and each contains key Tory target constituencies.

The Conservative fight against the Lib Dems centres on the southwest in local and general elections alike. Lib Dem control in Devon and Somerset is on a knife edge, with the Tories needing swings of 3% and 2% to secure a majority themselves. The new unitary Cornwall council will be elected for the first time.

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