After a crushing by-election win last week in Clacton, where the UK Independence Party picked up its first MP thanks to a Tory defection, a poll taken by Britain's Mail on Sunday put the party's support nationally at 25 per cent. That figure is almost 10 times its standing at the last general election. If repeated in next year's election, UKIP would hold more than 100 seats and wield a powerful hand in a coalition government – or hand the win to Labour. Prior to Clacton, most pollsters tipped UKIP for a single-figure result next May. Now Farage has muscled his way, through weight of numbers, into at least one of next year's pre-election TV debates – and wants in on the rest. The response has been panic. Prime Minister David Cameron has promised "one last go" at applying what is being called an "emergency handbrake" to EU immigration – which would require the OK from Brussels. "People recognise that Britain is an open, tolerant country, but immigration in recent years has been too high," Cameron told a town hall meeting in the electorate of Rochester and Strood, where a by-election next month could give UKIP its second body in the Commons.

Senior Tories are proposing what's being called an "Australian-style points system" for immigrants – in direct imitation of one of Farage's favourite policies. Labour leader Ed Milliband is also being pressed to get tougher on immigration, after his party narrowly headed off a UKIP challenge in a Greater Manchester seat that's traditionally considered its heartland. UKIP was "tapping into a seam of discontent and despair that Labour cannot — and will not — ignore", Milliband vowed in a column for The Observer. Farage relishes being on the outer. It's a role he's practised for years in Europe: "I am the atheist inside the temple," he told me last year in Strasbourg. "I am the heretic ... I am the devil, they're all focusing their hate on me." He and his party have long wanted to transplant this attitude to the UK – and now it is taking root.

YouGov pollster Peter Kellner says he's seen it all before – and that means it could happen again. For decades the Liberal Democrats were the insurgents, collecting the protest votes. Now they are in government. "UKIP in some ways was a replacement for the Liberal Democrats, as a party that people could vote for if they are irritated or disenchanted with the major political parties," Kellner says. Many times the Lib-Dems would win a by-election, their vote would briefly surge, yet at the general poll they would wither back down to a handful of seats. It took decades to reach the government benches. But UKIP is unusual. They have won one by-election through a Conservative defector, and may win another. There could be a domino effect, keeping the party's momentum through to the election. Kellner predicts up to 10 seats for UKIP at the next election – he thinks triple figures is "fantasy land" – but perhaps enough to make it the kingmaker in a conservative coalition, or the splitter that hands Downing Street to Labour.

And that could mean the end of British politics as we've known it for a century, he says. Since its birth in the 19th century, the Conservative Partyhas been "always driven by two distinct impulses", Kellner says – the capitalist instinct of trade, commerce and enterprise; and the patriotic instinct of nationalism, tradition and the flag. "When the two impulses worked together the Conservative Party was always invincible," he says. "When these two were at war with each other the Conservatives have been out of office. "This is an existential threat. If UKIP survives and puts down roots, it will formalise the separation of these two strands of right-of-centre politics." Kellner says the major parties are making a mistake by responding to UKIP by addressing its stated concerns of EU membership and immigration.

"They are symbolic of something more fundamental, an economic and social insecurity, a feeling that Britain is changing in a way that people don't like," he says. To address the symptoms is not to cure the disease, but only to confirm people's fears. And then there is Farage himself. Kellner says he is an "anti-politician … a vigilante manning the barricades, not a potential leader of the country". But right now, in the wake of the Iraq war and the MP expenses scandal, Farage, this public school-educated former City trader, is a vigilante the country wants to hear more from.