The UK Independence Party, which opposes the European Union, won by a landslide its first elected seat in Parliament on Friday and was a close second in another vote, setting itself up as a threat to the country's two main parties in a national election next year.

UKIP, which wants a British EU withdrawal and strict curbs on immigration, was projected to do well in both votes. But the unexpectedly wide margin of its victory in the seaside town of Clacton and its strong performance in an election in northern England, which it also almost won, took officials by surprise.

In Clacton, it won 60 percent of the vote after the sitting parliamentarian for Prime Minister David Cameron's Conservatives defected to UKIP, which didn't put up a candidate for the area when it was last contested in 2010.

In Heywood and Middleton, in northern England, a traditional stronghold for the opposition Labour party, UKIP got almost 39 percent of the vote, up from less than 3 percent in 2010.

"There is nothing that we cannot achieve," Douglas Carswell, Clacton's new UKIP member of parliament, told supporters.

Quoting Abraham Lincoln's Gettysburg Address and the words of John Wycliffe, a 14th century dissident translator of the Bible into English, Carswell said he backed "government of the people, by the people, for the people".

"The governing can no longer presume to know what is right for the governed," he said immediately after he was declared the winner. "Crony corporatism is not the free market. Cozy cartel politics is not meaningful democracy. Change is coming."

There is little prospect for now of UKIP's winning more than a half-dozen of Parliament's 650 seats in a national election next May.

But its success threatens to split the center-right vote and chip away at the traditional left-wing vote as well, making it harder for any one party to win an outright majority.

That increases the likelihood of a hung parliament, another coalition government and potential political instability in the world's sixth-largest economy.

Alarm clock

Grant Shapps, chairman of Cameron's Conservatives, said UKIP's success, if repeated next year, would hand victory to Labour leader Ed Miliband.

"This is an alarm clock moment. This is a stark message," he told BBC radio.

"If what has happened last night were repeated in 210 days at a general election, and you saw Conservative become UKIP seats, what you'd have is Ed Miliband in government."

UKIP's success is likely to raise pressure on Cameron to become anti-EU, three years before a referendum on EU membership that he has promised to hold if re-elected.

Carswell defected from Cameron's Conservatives in August, triggering Thursday's Clacton vote.

He switched allegiance because he said he doubted the prime minister's determination to reform the EU.