* UKIP leads Britain's European election - partial results
* Main parties dealt blow in EU vote
* Anti-immigration UKIP party siphons off support
* Immigration tops voter concerns in UK
With 56 out of 73 seats declared, the UK Independence Party, which wants to pull Britain out of the European Union, had won 28.6 percent of the vote, ahead of both the Conservatives and Labour while the Liberal Democrats had won just one seat.
If 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, who had promised to unleash an electoral earthquake, 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 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.
"The Conservatives will certainly be having a huge debate about the (EU) renegotiation strategy and the Labour party are going to begin to ask themselves very strongly why they have not matched the Conservative promise of a referendum," Farage said.
With over three quarters of seats declared, but results in London still unclear, UKIP had won 22 seats, the Conservatives 16 seats and Labour 14. The Liberal Democrats won one seat, the Green party two, and the Welsh Plaid Cymru party won one.
UKIP has said it will contest up to 30 seats in the national election, threatening to split the centre-right vote and make it more difficult for Cameron's Conservatives to win re-election.
The Liberal Democrats, the junior partner in the governing coalition, had won only one seat, and were facing the prospect of being all but wiped out in the European Parliament, putting its leader Nick Clegg under pressure.
UKIP's share of the vote in the European elections was up around 12 percentage points from the last European elections five years ago, partial data showed.
Conservative Eurosceptic lawmakers are likely to increase their calls for Cameron to bring forward his promised EU referendum by a year.
Earlier 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 and included 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 aimed at reminding 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.