PM Cameron and rival Miliband traverse country in scramble for votes but no party is expected to win a majority.
Britons are gearing up for elections seen by analysts as the most unpredictable in living memory, with no party expected to win a majority.
The Conservative party and Labour are the major parties contesting Thursday's election, but several smaller blocs are on the rise, marking a shift to a type of fragmented politics that is more familiar in other parts of Europe.
Deputy Prime Minister Nick Clegg, the leader of the centrist Liberal Democrats, has even warned of another election this year if an unstable minority government takes power.
On the final day of campaigning on Wednesday, Conservative Prime Minister David Cameron and his chief Labour rival Ed Miliband embarked on tours of the country in a scramble for votes.
A Conservative victory could raise the risk of Britain exiting the European Union because it would mean a membership referendum, while some experts warn that a Labour win could spread unease among investors.
Polls open at 0600 GMT and close at 2100 GMT, with exit polls published immediately after that and the first results coming in from around midnight.
Vote tallies for the 650 seats will be announced during the night and final results are not expected until Friday afternoon.
Tens of millions of people are registered to vote, and nearly 4,000 candidates are in the running for parliament.
'Poll of polls'
Ballots will be cast in around 50,000 polling stations dotted around the country, including in unusual places like pubs, caravans and even garages.
If the election results are not decisive as widely expected, negotiations between the parties could start immediately, although they may be delayed by ceremonies for the anniversary of the end of World War II.
The latest BBC "poll of polls" average puts the Conservatives at 34 percent, followed by Labour at 33 percent, the anti-EU UK Independence Party (UKIP) at 14 percent and the Liberal Democrats, who are currently junior members in a governing coalition with the Conservatives, at just 8 percent.
But the percentage breakdown is a poor indicator of the final tally in Britain because of the first-past-the-post system, which counts the results only in individual constituencies, not the overall vote share.
Negotiations to form a government will likely be complicated, and a heated debate has broken out about the potential legitimacy of a coalition given that the party that wins the most seats may not end up governing.