LONDON/SOUTHAMPTON (Reuters) - Britain's anti-EU UKIP party swept to victory in European elections, riding a tide of Euroscepticism and anxiety about immigration to beat Prime Minister David Cameron's Conservatives and the opposition Labour party, almost complete results showed.
With 64 out of 73 seats declared, the UK Independence Party, which wants to pull Britain out of the European Union, had won 23 seats, ahead of the Conservatives and Labour on 18 seats each. The Liberal Democrats had won just one seat, putting it in fifth place behind the Green party on three seats.
If, as expected, the pattern is borne out by full results, it will be the first time UKIP has won a national election and the first time such an election had not been won by either the Conservatives or Labour since 1910.
UKIP leader Nigel Farage hailed the win as a watershed that would pile pressure on the leaders of Britain's three main political parties to adopt more Eurosceptic policies.
"The people's army of UKIP have spoken tonight and delivered just about the most extraordinary result that has been seen in British politics for 100 years," Farage said in the port city of Southampton after his re-election to the European Parliament.
Farage said that UKIP would now target seats in Britain's national parliament in Westminster and that the 20-year-old party could hold the balance of power if, as polls suggest, neither the Conservatives nor Labour wins outright next May.
"We will go on next year to a general election with a targeting strategy and I promise you this: You haven't heard the last of us," Farage said as his supporters quaffed champagne around him.
Cameron has promised to try to renegotiate Britain's relationship with the EU if re-elected next year before offering Britons an in/out EU membership referendum in 2017. Labour has said it is unlikely to hold such a vote if it wins power.
UKIP wants an immediate referendum and for Britain to leave the 28-nation bloc, arguing the country will be more democratic and prosperous outside the EU.
Though the Conservatives and Labour were tied on seats, Labour's percentage share of the vote was a little higher, giving it second place for now. If it finishes with just a small gap separating it and Cameron's party, some will argue it should be polling much more strongly if it hopes to win a 2015 national vote.
The results were a disaster for junior coalition partner the Liberal Democrats - it won 11 seats five years ago - and were likely to increase pressure on Nick Clegg, its leader, who is already facing calls to quit.
UKIP has said it will contest up to 30 seats in the national election, threatening to split the center-right vote and make it more difficult for Cameron's Conservatives to win re-election.
UKIP's share of the vote in the European elections was up around 11 percentage points from the last European elections, partial data showed, more than any other party.
Conservative Eurosceptic lawmakers are now likely to increase their calls for Cameron to bring forward his promised EU referendum by a year.
On Sunday, Cameron's Conservatives promised new measures to curb immigration from the European Union in an effort to appease UKIP voters after losing hundreds of seats in local polls last week.
UKIP siphoned most of its support in those elections from the Conservatives, who lost over 200 seats, a trend that partial results showed was being repeated in the European vote.
Public discontent about rising immigration, particularly from poor EU countries like Romania and Bulgaria, is one of the main factors driving support for UKIP.
RISING EU IMMIGRATION
Data last week showed the number of EU citizens moving to Britain rose 27 percent in 2013, an awkward statistic for Cameron who has promised to reduce net migration to the "tens of thousands" by May next year, a target he is on course to miss.
UKIP argues immigration is putting unacceptable strain on public services and changing communities' identity. It wants to seriously restrict inflows.
Home Secretary Theresa May, the interior minister, said on Sunday the government planned to increase fines for employers who do not pay the minimum wage, a move aimed at discouraging them from hiring immigrants willing to work for less.
The Sunday Telegraph newspaper said new immigration measures would be unveiled on June 4, including a law to deport unemployed EU nationals after six months and a "wealth test" to stop people coming from poor EU member states.
In a move to remind voters of the Conservatives' promise to hold a referendum on leaving the EU by the end of 2017 if re-elected next year, one of their lawmakers will try to introduce a bill making such a referendum legally binding for all parties.
Neither Labour nor the Liberal Democrats are expected to back it.
(Reporting and writing by Andrew Osborn and Guy Faulconbridge. Editing by Mike Peacock)