The UK Independence Party hailed a rise of two thirds in its membership in the past 12 months as it gears up for European Parliament elections in May.
Nigel Farage's eurosceptic party said it was finishing 2013 with 32,500 members - up from 19,500 at the start of a year which has seen its profile rise significantly.
It contrasts with dwindling grassroots support for the established Westminster groupings - although Ukip still has only a fraction of the members of the two main parties.
Conservative constituency membership has almost halved since David Cameron became leader in 2005 - from 253,600 to just 134,000.
And while Labour's membership rose significantly when Ed Miliband became leader and remains above its 2009 level, it has steadily slipped back to stand at 187,537.
The Liberal Democrats said a sharp increase in new members in the final quarter of the year - reversing a slump in the first half - was proof of a "feel good factor".
But its total of around 43,000 remains more than a third lower than the 65,000-plus levels it enjoyed before joining the Conservatives in coalition government in 2010.
Ukip chairman Steve Crowther said: "The narrative in Westminster seems to be that falling party membership is inevitable.
"Well in Ukip's case the opposite is true: our membership is growing faster than ever.
"In May alone, the month that the council elections were held, Ukip membership went up by 2,500. We will now aim to continue that momentum and growth as we head into next year's European Elections and the 2015 general election as well."
Farage - who has predicted a political "earthquake" in May when his party is tipped by some to win the election - said it was re-engaging people who had "given up on politics".
"Many of our members are those who have never been a member of any political party before but who see that it is Ukip who is challenging the establishment on issues such as open door immigration, grammar schools and green taxes."