With no party expected to win a majority and several smaller blocs on the rise, the election could also mark 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 Wednesday, Conservative Prime Minister David Cameron and his chief Labour rival Ed Miliband embarked on frenetic 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.
The Conservatives and Labour have sharply different views over whether to continue austerity cuts in the world's fifth-biggest economy, which have slashed the deficit but also led to widespread social pain.
At least one thing is for sure -- expected massive gains by Scottish nationalists will transform the British political scene and make the prospect of independence for Scotland a far more likely one.
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.