Polls have closed in European and local elections which could signal a new era in British politics if the results produce the "earthquake" predicted by Nigel Farage and his UK Independence Party.
Votes were cast throughout the UK for the European Parliament contest to return 73 MEPs, while more than 4,000 council seats at 161 English local authorities, including the London boroughs, and those in Northern Ireland are also up for election.
With a year to go until the general election, the stakes are high and all parties will be studying the results for indications about their prospects in 2015.
Counting will take place in some of the local elections overnight, but the European declarations will not begin until Sunday - meaning a wait to see if Ukip's opinion poll lead translates into first place in the contest and a nervous few days for the Liberal Democrats who face the prospect of being wiped out.
Labour, the Conservatives and Lib Dems will study the results for clues as to whether Ukip is developing into an electoral force which could hold the balance of power at Westminster in 2015 - or if Mr Farage's bubble will burst and disillusioned voters can be tempted back after making a protest at the ballot box this year .
A victory for Ukip could present 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.
Official figures showed the Tory promise to reduce immigration to the "tens of thousands" by May 2015 is moving further out of reach as new data from the Office for National Statistics (ONS) showed net migration increasing to 212,000 in the year to December, from 177,000 the previous year.
Labour has claimed a good night would yield an extra 200 councillors, although independent commentators have suggested Ed Miliband's party should be hoping for up to 500 gains.
The party has been pouring 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 around 22 MEPs instead of the current 13.
A senior source appeared to concede that Ukip could top the polls and 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 have acknowledged they are facing a "difficult" night which could leave it with no representatives in the European Parliament.
The party is set for an electoral mauling in the council contests and is facing a fierce battle to avoid losing control of some of its local authorities, including Kingston-upon-Thames, which includes the Westminster seat of Cabinet minister Ed Davey.
A Lib Dem source has suggested the party's hopes rest on getting their vote out in the strongholds where they have Westminster MPs, so Kingston will be an indication of the success of that strategy.
Police were stationed at some polling stations in London, including in Tower Hamlets where the controversial mayor Lutfur Rahman is standing for re-election, and where the contest in one ward has been postponed due to the death of a candidate.
The electoral marathon will see some votes still being counted on Monday.
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.
Final results in Scotland will not be available until Monday morning, because of opposition to Sunday working in Comhairle nan Eilean Siar - Western Isles.
But - at the same time - counting will be getting under way in Northern Ireland.
The Prime Minister said he was "proud" of the Conservative campaign "whatever the results".
In a message on Twitter, Mr Cameron said: " To all the Conservatives who campaigned these past few weeks: thank you.
"Whatever the results, I'm proud of the campaign we fought together.
"And, with the polls now closed, I'd also like to say thank you to everyone who voted Conservative today. Your support is hugely appreciated."
In an email to supporters, Mr Cameron said the choice in 2015 would be between the Tories and Labour - with no mention of Ukip but an appeal to "every Conservative in the country" to vote Tory.
He said: " Now we must continue our focus on the big issues facing Britain.
"We need to carry on reducing the deficit to safeguard our economy for the long term.
"We need to keep on backing businesses so they can create even more jobs and give even more people the security of a pay packet each month.
"We need to continue to control immigration and fix the welfare system so our economy delivers for those who want to work hard and play by the rules.
"In 2015, the British people will face a choice: between a long-term economic plan which is securing a better future for Britain and the same old Labour Party offering only more spending, more borrowing and more taxes.
"If we're going to win that election, we need the support of every Conservative in the country."
Mr Miliband also used Twitter to thank party activists for their efforts and to look ahead to the general election.
He said: "Thank you to every Labour supporter out campaigning today (especially those who got soaked!).
" Today, we campaigned for a Britain where hardworking people are better off. Tomorrow the campaign begins to win that same fight in May 2015."
A Labour source said the party was "confident of doing well in key areas" and predicted turnout to be "in the low 30s".
In 2009, turnout for the European elections was 34.7%.