Tomorrow’s elections to the European Parliament could produce “the most volatile voting patterns in living memory”, with the British National Party winning as many as five seats and the Greens also making big gains, according to a report just published.

The report by think-tank Democratic Audit said that conventional opinion polls could be misleading in an election where turnout is likely to be low and where regional variations may be crucial.

Controversy over MPs’ expenses and the recession could combine with the proportional voting system used in the Euro-elections to produce a significant drop in support for the main parties and major gains for smaller groupings, said think-tank director Stuart Wilks-Heeg, the author of the report What are European Elections For?

“This promises to be one of the most unusual elections we have ever seen,” said Dr Wilks-Heeg.

“The voting system, the current political climate, and regional concentrations of support could see smaller parties fare exceptionally well on Thursday.

“If turnout remains at just under 40% and regular voters switch support away from the main parties, rather than stay at home, we would see seats won by the Greens and BNP. We don’t expect Ukip to improve on their high water mark of 2004.”

The report said that the D’Hondt voting system used in Euro-elections in England, Scotland and Wales favours smaller parties, whose share of the vote has steadily increased since 1994.

Based on closed ‘lists’ and dividing the leading party’s share of the vote in each counting round, it means that a party with around 10% support can secure an MEP in an eight-seat regional constituency.

The report set out five possible scenarios for tomorrow’s election, depending on how many voters desert the main parties for a smaller rival and how many mark their protest by staying at home.

In the scenario most favourable to the smaller parties, Democratic Audit predicted that the BNP could win a single seat in as many as five English regions and Greens gain an additional two seats while successfully defending their existing two.