Election 2015: The pundits have less idea than ever who will win this lottery election

The arrival of six-party politics and an idiosyncratic general election result will “revive the debate about electoral reform”, according to one of Britain’s foremost polling experts.

Whereas elections used to be fairly predictable, with one of the two main parties prevailing and the Liberal Democrats rarely winning more than a handful of seats, this May’s election is agreed by many to be the most unpredictable in history, with the Scottish National Party set to take Labour seats and Ukip potentially getting the third most votes.

In “The Lottery Election”, a report commissioned by the Electoral Reform Society (ERS) and published tomorrow, Professor John Curtice identifies a number of situations where a slight shift in the vote could result in a huge difference in the composition of the House of Commons.

The situation is so delicate that the SNP could secure a small number of seats – or more than 50, which would make it the third-biggest party.

Labour could also secure an overall majority with a lower share of the vote than any single- party government in history. Unequal constituency sizes mean that Tories might need a lead of at least seven points to form a Government without the support of another party.

The report says: “Despite the continued use of an electoral system that is supposed to ensure that one party has an overall majority, a hung Parliament looks as likely an outcome as an overall majority, if not more so.”

Research by public relations firm Hanover Communications and polling group Populus yesterday put the chances of a second consecutive hung Parliament at 94.5 per cent. This is remarkable given that the current Government is the first coalition since the Second World War, and that the last election before 2010 to fail to establish an outright winner was in 1974.

“A second failure of the First Past the Post system to deliver an overall majority ... could in itself be expected to revive the debate about electoral reform,” the report says. Professor Curtice found that a fairly traditional analysis of the polls would suggest a 43-seat Labour lead over the Tories, with just 13 SNP and Plaid Cymru victories. However, the changing nature of SNP support means this could be a 30-seat Labour lead, with the two nationalist parties winning 56 seats.

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Professor Curtice’s model suggests that even with 13 per cent of the vote, Ukip could outpoll the Lib Dems but end up with far fewer seats. He has also established Norwich South, currently held by Lib Dem Simon Wright, as the Green Party’s best hope for a second MP.

Darren Hughes, the ERS’s deputy chief executive, hopes that the chaotic election will mean the current voting system is “revealed for what it is – a relic from another age”.

The ERS wants changes to the voting system to be part of an overhaul of British politics established by a constitutional convention later this year. Scotland is set to get a raft of powers in a fresh devolution package, and the Conservatives want English MPs to have what amounts to a veto over laws that only affect England.

Labour and the Lib Dems want a constitutional convention, which has the support of the ERS and Unlock Democracy. These parties and groups, as well as the Greens and academics, met in Parliament last month to start thrashing out how this convention would work.