London: Once a marginal group of anti-Europeans, the UK Independence Party (UKIP) is now a force to be reckoned with after its best-ever national election result spooked Prime Minister David Cameron’s Conservatives.

UKIP took 28 per cent of the vote in Thursday’s by-election in the southern English seat of Eastleigh, coming second behind junior coalition partners the Liberal Democrats and pushing the Tories into a humiliating third place.

Although the anti-Brussels party has yet to win a seat in the British parliament, the result is its best in a string of good performances in mid-term votes in recent months.

UKIP leader Nigel Farage said it was part of a trend.

“What happened here in Eastleigh was not a freak result. Something is changing. People are sick and tired of having three social democrat parties that are frankly indistinguishable from each other,” he said.

The reasons for UKIP’s success in Eastleigh are manifold, and not limited to the anti-European cause around which it was created. Immigration, the economy and general dissatisfaction with the government all played a part.

But the result has hit the Tories hard, not just because they should have done better in Eastleigh, where the vote was sparked by a Lib Dem scandal, but because it feeds into growing fears that UKIP is eating away at its support.

The Tories have not won a parliamentary majority since 1992 and many in the party fear UKIP is syphoning off the anti-European, anti-immigration vote that they will need to ditch their Lib Dem coalition partners and win outright in 2015’s general election.

Cameron’s promise in January to renegotiate Britain’s membership of the European Union and put it to a referendum by 2017 was widely seen as an attempt to calm eurosceptics in his party and fend off the threat from UKIP.

After the result in Eastleigh, outraged senior Tories lined up to warn Cameron - who once described UKIP as full of “fruitcakes, loonies and closet racists” - that they must reach out to traditional voters.

Cameron described the by-election as a protest vote, but vowed to try to “win those people back”, although he insisted he would not change his policies.

However, academic research suggests that while UKIP and the Conservatives share many of the same policy goals, it is too simplistic to say that UKIP is simply taking away disaffected Tories.

In Eastleigh, the Tory candidate Maria Hutchings was described as “more UKIP than UKIP”. She is a eurosceptic who said she would vote for Britain to leave the EU and who opposes Cameron’s plans to legalise gay marriage.

Robert Ford, an academic at the University of Manchester who has done extensive research on UKIP’s support, said the party’s voters hail from across the political spectrum, but are often working class and economically insecure.

“The primary issue for those supporting UKIP is immigration”, an issue on which the government has talked tough but is often powerless to change, he said.

“The broader outlook is a disgruntlement with modern Britain. They want to turn back the clock. These are older voters who don’t like the face of Britain today.”

While UKIP is likely to do very well in next year’s European Parliament elections, he said their support is too disparate - a “coalition of the disgruntled” - to make a real dent in national elections.

Critics also warn that UKIP’s populist policies are impractical, while the party lacks structure and any big names beyond the oddly charismatic Farage.

Peter Kellner, the president of opinion polling firm YouGov, agreed that UKIP belonged to a “long tradition of mid-term insurgents” who would not have a lasting impact - unless the Conservatives overreact.

“If enough of them panic and engage in internecine strife, then the party really could destroy its chances of victory in the 2015 general election,” Kellner said in an online commentary.

“They should hold their nerve and wait for the UKIP bubble to collapse in 2015, when it struggles to compete under a first-past-the-post system in an election to choose the nation’s government.”