Voters are going to the polls in the closely-contested Eastleigh by-election.
The Hampshire constituency has previously been held by the Conservatives and is one of the seats viewed as key to the party winning an outright majority at the 2015 general election.
But the Liberal Democrats are desperately fighting to hang on to it to give a vital morale boost to the grassroots after support hit the doldrums on entering coalition - though their campaign has been marred by allegations that the party failed to take proper action to deal with claims that senior peer Lord Rennard sexually harassed party activists.
Opinion polls suggest the race remains very tight.
The by-election will declare a replacement for Lib Dem ex-Cabinet minister Chris Huhne, who resigned in disgrace after admitting perverting the course of justice.