The Conservatives and Labour were left struggling to formulate a response to the challenge of Ukip after Nigel Farage’s Eurosceptic party took its first Westminster seat and came within an ace of gaining a second.

After a momentous night for British politics, a triumphant Farage claimed his “people’s army” could hold the balance of power in the next Parliament, having made gains at the expense of the two main parties in by-elections held in their traditional heartlands.

The whole of British politics has been shaken up

Prime Minister David Cameron said the general election in May would be “the most important in a generation” – warning that voters who switched support from the Tories to Ukip risked handing power to Labour.

Labour leader Ed Miliband acknowledged that his party needed to “reach out” to dis­affected voters after seeing one of his northern strongholds almost fall to the Ukip insurgency – a result which threatened to re-open the issue of his leadership.

Newly elected Ukip MP for Clacton-on-Sea in Essex, Douglas Carswell (second right), celebrating after winning the by-election. Photo: Stefan Rousseau/PA Wire

In a night which underscored Ukip’s ascent from the political fringes to centre-stage, Tory defector Douglas Carswell cruised to victory in Clacton in Essex where he had stood down as MP to fight the seat as a Ukip candidate.

And in the traditional Labour stronghold of Heywood and Middleton in Greater Manchester, Ukip came within just 617 votes of overturning a 6,000 majority after demanding a recount.

Farage said the results would provide a springboard for Ukip to take a second seat in the forthcoming Rochester and Strood by-election, where another Tory defector – Mark Reckless – has also stood down to run for Ukip.

“The whole of British politics has been shaken up in a way that the complacent Westminster class could never even have contemplated,” he told BBC News.

“Something big is happening here. People want change. They have had enough of career politicians in three parties who don’t even understand the problems they face in their everyday lives. People want real change. We’ve got a chance here in a general election next year that is likely to be very tight, in an election in which no one party is likely to have a majority – if Ukip can keep this momentum going, we could find ourselves next May in a position where we hold the balance of power.”