The next general election could be decided by just 8,000 voters, the Electoral Reform Society has claimed.

The organisation published the study to mark what the Conservatives have dubbed "the election day that never was" - the likely November 1 date of a poll until prime minister Gordon Brown decided against going to the country.

According to the study, just 8,000 swing voters in 25 key Labour marginals will decide if Labour loses its overall majority.

The society, which campaigns for a move to proportional representation, said the influence of such a narrow group of voters was the result of the first-past-the-post electoral system and boundary changes which favour the Tories.

Chief executive Dr Ken Ritchie said the numbers showed the country "simply did not need another unreconstructed election".

"Brown's opponents may be claiming that a new government needs a mandate. Well a mandate delivered by 8,000 people in the swing seats is no mandate at all," he added.

"We call on the prime minister to go back to his route map to constitutional reform."