Sunder Katwala is Director of British Future.

The 2014 European elections may take place across a continent, but they will be fought and understood as a series of national political contests. The big question in British politics will be what it means for 2015. Party leaders and their MPs, plus activists and commentators will pore over the results to work out who is heading for triumph or disaster less than a year later.

The problem is that European Elections are a very poor guide to general elections. As the analysis in British Future’s new report, ‘The Rise and Rise of the Outsider Election’ shows, European elections have become less and less like the general elections that follow them since the switch to a PR European election in 1999.

The voters are different. About 15 million General Election voters will be missing this May, but will turn up when it is time to elect a government in 2015. This much smaller and narrower European electorate is older, whiter and more vocally Eurosceptic. As the US Republicans found between 2010 and 2012, a winning message which resonates with the narrower electorate in a mid-term contest can leave a party dangerously adrift when a broader cross-section of a changing society turns up to choose a government.

Those who do vote are much more open to choosing smaller parties. Outsider parties – those with no seats in the House of Commons – have since 1999 averaged 24 per cent of the European Elections vote, falling to four per cent in the General Election. The joint Conservative and Labour European share was 48 per cent in 2004 and 44 per cent in 2009, but they have continued to share two out of three General Election votes.

The only major party not to see its general election vote share go backwards in a European Election since the switch to PR were William Hague’s Conservatives in 1999. A major party ran on a UKIP-style agenda and so got an outsider party result – a European election triumph and a general election disaster.

Many of these differences in voting (and non-voting) behaviour are because most voters don’t care much about this election. Lord Ashcroft has written “Voter readily distinguish between elections that matter and those that don’t,” noting that his focus group participants cheerfully compared European elections to the Eurovision song contest. But that isn’t the whole story. For some voters, the European elections really matter. This is also the Eurosceptic World Cup final. Asked which 2014 events mattered to them by Ipsos-Mori, voters put the European elections in last place. But UKIP voters disagreed, with twice as many choosing the European elections as the football World Cup.

The question nagging at Conservative minds: could a governing party really come third, yet be re-elected less than a year later? The analysis of past voting history suggests there is no reason why not.

Governing parties have gained 13 per cent in vote share between the European and General Elections since 1999. This suggests a 2014 benchmark for David Cameron would that 23 per cent of the May 2014 vote could be a fair par score if he was to return to 36 per cent in 2015. He might need to aim higher to secure a majority.

UKIP has a strong chance of winning the European elections but a much greater challenge to translate this into General Election success. They won over 16 per cent in both 2004 and 2009, but fell to 2-3 per cent within a year. Even as the pool of voters doubled, they shed almost two million votes.

Unless UKIP can sharply improve its past retention of votes between general and European elections, even a 30 per cent share and 4.5 million European votes in 2014 would slip away to a six per cent General Election share, on about 1.8 million votes.

So UKIP are favourites for the European election but remain odds-on with the bookmakers at 8/15 to fail to win even a single Commons seat in 2015. UKIP could match the Greens and enter the Commons for the first time, but have little realistic chance of having as much presence in the Commons as the Welsh Nationalists, with their three MPs.

For the major parties, the question is how to attract back UKIP protest votes, without repelling other potential voters. David Cameron will want to re-engage that section of UKIP voters – around half of them – who are open to what a governing party can offer on Europe, immigration or, for many, pensions, without turning off other voters. The Conservatives also need to attract more votes from four of the groups least likely to vote UKIP – women, graduates, 2010 LibDems who are pro-Coalition – to win a majority in 2015.

When considering the 2014 exit polls, the political leaders will need to think about the majority of those who didn’t vote, as well hearing the message from those who did.

‘The rise and rise of the outsider election: What will the 2014 European election mean for British politics?’ is published by British Future on Tuesday. See