They are meant to be about the future of Europe and the delivery of vital local services such as schools and social care. But in truth it will be the future of Gordon Brown that will be the main question hanging over the European and local elections on 4 June.

If Labour does badly, the recently revived muttering and perhaps even the plotting about the Prime Minister's future can be expected to thicken. The expectation that David Cameron is on course for victory in a general election next year will grow. If, on the other hand, Labour avoids disaster Mr Brown will be safe and on the Labour benches hope will be rekindled that Mr Cameron could yet be denied.

At first glance the European elections would seem to be the more severe of the two electoral tests for Mr Brown. The whole country will get the chance to vote in a straightforward proportional representation election. Labour's success or failure will be unambiguously recorded for all to see.

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In contrast local elections are only taking place in shire – and thus mostly Tory – England. Only 27 county and seven unitary councils (including five new ones) are at stake. Any shortcomings in Labour's performance would seem capable of being dismissed on the grounds that little of national significance can be read into such unrepresentative results.

In truth, Mr Brown may well have less to fear in the European elections than he does in the local contests. This is for one simple reason. When the country last voted in European elections in 2004 Labour performed abysmally. It won less than 23 per cent of the nationwide vote, not least because the party was beginning to pay for the unpopularity of the Iraq war. Yet just 12 months later Tony Blair still went on to win a historic third term. Even the most recent opinion polls still suggest the party stands at 27 per cent in Westminster voting intentions. Getting 23 per cent or thereabouts ought not be too difficult. And so long as the result is not significantly worse than 2004, Labour's spin machine will be able to argue it can recover in 2010 just as it did in 2005.

Mind you, European elections have the habit of throwing up the unexpected. In 1989 the Greens famously won as much as 15 per cent of the vote, largely at the expense of the Liberal Democrats. Five years ago, Ukip that took the country by storm, securing as much as 16 per cent of the vote, with the Conservatives (and again the Liberal Democrats) losing out.

Labour's fear is that this time it will be the BNP that makes a sudden breakthrough and Labour will take the hit. Nick Griffin's party does best in working-class areas with high ethnic-minority populations in the North of England and the Midlands – in other words in traditional Labour territory.

But if past precedent is kind to Mr Brown in the European elections, it could be a serious source of embarrassment to him in the local elections. For this particular round of elections was last contested on general election day 2005.

True, people did not vote in those local elections in exactly the same way they did in the general election – the Liberal Democrats fared better and Labour and the Conservatives rather worse.

But even so, the results will provide a direct measure of how much Labour has fallen from favour since its last election victory.

Moreover, the few scraps of power Labour does have in shire England are all vulnerable. The party is defending just four councils: Derbyshire, Lancashire, Nottinghamshire and Staffordshire. All four will be lost if the swing against Labour reflects that currently registered by the opinion polls. And never before has a party suffered a complete wipe-out on local election day.

Those with long memories will recall the last and only time that Labour suffered defeat in Derbyshire and Nottinghamshire. It was in 1977 during the darkest days of the last Labour government under Jim Callaghan, a government whose demise heralded 18 years of Tory rule. So perhaps Labour MPs would regard losing those two counties again as the clearest sign of all that Mr Brown is leading them to serious defeat.

John Curtice is professor of politics at Strathclyde University