Voters across much of England will be electing local councillors, while elections will also be held for devolved assemblies in Scotland, Northern Ireland and Wales.
The coalition government, led by the Conservatives in alliance with the smaller Liberal Democrats, is expected to suffer a backlash from voters unhappy about tough austerity measures designed to tame a record peacetime budget deficit.
However, the referendum on whether to change the way members of parliament are elected has been the real source of tension between the coalition partners, who are on opposite sides of the argument and have traded insults during the campaign.
Voters are expected to reject change in favour of sticking with the long-established, first-past-the-post system, disappointing the Lib Dems.
Analysts believe the coalition, Britain's first since World War Two, will stick together because for either partner to force an early election would risk political suicide at a time when the economy is growing very slowly.
"I think they still need each other and will try to make this work," said Tim Bale, politics professor at the University of Sussex in southern England. "The atmosphere has changed but the parliamentary arithmetic has not," he added.
Markets want a stable government to see through a five-year plan virtually to eliminate by 2015 a budget deficit that had been running at more than 10% of national output.
"Miserable little compromise"
The Lib Dems insisted on a referendum on a switch to the Alternative Vote (AV) system, seen as more favourable to smaller parties, as a price for entering coalition last May.
But AV falls short of their ideal of a genuinely proportional voting system which they believe would end the domination of the British political system by two large parties - the Conservatives and opposition Labour.
Nick Clegg, Lib Dem leader and deputy prime minister, has denounced AV as a "miserable little compromise" in the past, and the electoral reform campaign has failed to excite Britons distracted by a series of public holidays and a royal wedding.
"I think this never was their cause in the first place," said Bale. "It's a hybrid system they couldn't really sell."
Most opinion polls put Labour, ousted from power a year ago, as the most popular party, while support for the Lib Dems has collapsed since they entered coalition.
Labour expects to perform strongly in the local polls in England but is struggling to overtake the pro-independence Scottish National Party (SNP) in Scotland.
Lib Dem activists were not panicking despite their current unpopularity.
"If we lose [the referendum] it will be difficult, but that is no reason to ditch the coalition," Simon Cook, a Lib Dem councillor in the western English city of Bristol, said earlier this week.
"Ultimately the work of the coalition and getting the country right is more important," he added.
(EurActiv with Reuters.)