By Andrew Osborn and Guy Faulconbridge
LONDON (Reuters) - Britain's anti-EU UKIP party has chalked up strong gains in elections to the European Parliament, polling more strongly than Prime Minister David Cameron's Conservative party and opposition Labour party in early results on Sunday.
With 15 out of 73 seats declared, the UK Independence Party, which wants Britain to leave the European Union, had won six seats, the Conservatives five seats and Labour four.
"The political earthquake that UKIP promised is firmly underway tonight," said Patrick O'Flynn, who was elected as a UKIP MEP for the East of England European Parliamentary constituency.
"The British people have spoken: they want control of their nation, they want some of their money back, they want control of their borders too. And any party who doesn't hear the message loud and clear is in for another shock less than a year from now when we fight again at the general election."
UKIP has said it will contest up to 30 seats in next year's national election, threatening to split the centre-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 over 13 percentage points from the last European elections five years ago, early data showed.
If the trend is maintained, the result is likely to put pressure on Cameron to step up a drive to limit intra-EU immigration and to repatriate powers from the EU if he is re-elected next year.
Eurosceptic lawmakers in his own party are also likely to increase calls for him to bring forward a promised EU membership referendum from 2017 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 analysts think is likely to be 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 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 Daily 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.
(Reporting and writing by Andrew Osborn and Guy Faulconbridge. Editing by Mike Peacock)