Polling has opened across Britain as millions of voters cast their ballots in European and local council elections.
The booths opened at 7am throughout the UK for the European Parliament contest to return 73 Euro MPs, while more than 4,000 council seats at 161 English local authorities and those in Northern Ireland are also up for election.
Labour, the Conservatives and Liberal Democrats will await the impact of the Ukip-effect as voters go to the polls following a campaign dominated by Nigel Farage's Eurosceptics.
Party chiefs and pundits will scan the results closely as they attempt to assess if the UK Independence Party is developing into a long-lasting force that will sway who holds power at Westminster after next year's general election - or if people can be tempted back after making a protest vote this year.
A poll by YouGov for The Sun projected that Ukip would win the European elections with 27% of the vote, with the Lib Dems back in fifth place.
Party insiders are braced for the possibility of the Liberal Democrats losing all of their 11 European seats, according to a leaked document, the Guardian said.
Bookmakers predict the Conservatives could be pushed into third place, with Ladbrokes reporting punters are backing Ukip to pick up the most votes in the Euro poll ahead of Labour.
Mr Farage's party hopes to challenge for first place in the European election after a strong showing in 2009 and in council contests and Westminster by-elections.
This scenario coming to fruition could provide Prime Minister David Cameron with a challenging 12 months in Downing Street as he seeks to appeal to the nation and keep the Eurosceptic wing of the Conservative Party content on issues including immigration.
The Government's attempts to slash net migration to the tens of thousands will come under the spotlight as official figures are released at the same time as voters go to the polls.
The election results will also be a clearer indication as to whether Mr Farage's party has successfully negotiated close media scrutiny of its candidates and policies.
A senior Labour source said the party hoped to gain up to 200 councillors, and had been ploughing resources into seats they will be targeting in next year's general election.
A good night would see Labour claim a 25% share in the European elections, giving Ed Miliband's party 22 MEPs instead of the current 13.
But the party admitted that European contests had been "historically difficult" for Labour and other centre-left parties across the EU.
The source also indicated that it would not be surprising if Ukip claimed the top spot and British politics entered a new era.
The source said: "We are now in an era of four-party politics but what we have got to do, and what we hope we are starting to do, is win where it matters in the local elections."
Labour was "ruthlessly" targeting its efforts at battleground seats in next year's Westminster contest, inspired by US president Barack Obama's strategy - masterminded by Labour's new guru David Axelrod - of focusing resources where they would have the most impact.
"We know we are going to have less money than the Tories, so we have got to make better use of the money and people we have got, and to use that in the most sensible way," added the source.
The party hopes the strategy will pay dividends in areas including Redbridge, Croydon and Cambridge.
Labour strategists will also be hoping to see the Tories failing to hold seats in marginal wards in Basildon, Peterborough, Southend and Swindon - the authority where Mr Miliband failed to recognise the name of the party's senior councillor during a campaign radio interview.
The pro-EU Liberal Democrats are braced for a "difficult" night as the results are returned.
The party is set for an electoral mauling in the council contests and could find it suffers a potential wipeout in the European vote after its fourth place in the 2009 poll.
Snap opinion polls also suggested Lib Dem leader Nick Clegg was considered to have lost in two televised debates with Mr Farage as his "party of in" pushed the EU case against Ukip, the "party of out".
A Lib Dem source has suggested the party's hopes rest on getting their vote out in the strongholds where they have Westminster MPs.
The biggest battlefield in England is expected to be London, with all seats in the capital's boroughs up for grabs.
Polling stations are open throughout the day and some councils will count overnight, although many will declare during Friday.
Counting of the European Parliament votes starts on Sunday but no announcements can be made until 10pm under rules barring declarations until polls have closed all over the EU.