Polls have opened to millions of voters in the most uncertain general election for decades, with no party on course to emerge a clear winner.

Last-ditch appeals by party leaders did nothing to break the opinion poll deadlock and left the country facing the prospect of another hung parliament.

But with millions apparently still undecided or open to changing their minds, the likely new Westminster balance of power remained highly unclear.

Among the last set of polls, three showed the main parties level pegging, three had the Conservatives in front by a single point and one gave Labour a two-point advantage.

David Cameron said the way voters cast their ballots would "define a generation" and appealed for more time to build a better Britain, warning a Labour government would be "held to ransom" by Scottish nationalists.

But Ed Miliband accused him of hiding the truth about deep spending cuts that posed a "real and present danger" to families' finances and urged people to bring an end to "five years of unfairness, five years of failure".

In one of the biggest pre-election polls, a YouGov survey of 10,000 voters for The Sun had the main parties on 34 per cent each - but with a significant 17 per cent saying they were yet to make up their minds - a figure put as high as 25 per cent in a ComRes poll for ITV and the Daily Mail.

In the past such a tie would have been enough to propel Ed Miliband into Downing Street but an SNP surge in Scotland threatens to rob Labour of dozens of its traditional strongholds north of the border and of the chance to govern alone.

Nick Clegg - who faces a fight to hold on to his own Sheffield Hallam seat - urged voters to stick with the Liberal Democrats as the only party able to provide a "stable" influence to a Tory or Labour administration.

 Ukip leader Nigel Farage predicted many undecideds would swing behind the Eurosceptic party as it seeks to translate regular third places in national polls into an influential Commons presence in any post-election negotiations.

The Green Party will also hope to increase its parliamentary presence, heavily targeting three seats in a push to underline the increasingly fractured political make-up of the electorate.