The UK Independence Party formed in 1993 but in its early days was overshadowed by the well-financed Referendum Party led by Sir James Goldsmith which was wound up soon after the 1997 election.
Mr Knapman is standing down after four years as leader
UKIP claims not to be 'anti-European' - it just does not want Britain to be part of the EU.
The party says that the increasing numbers of laws coming from Brussels are threatening Britain's independence.
UKIP particularly objects to Europe's interference in tax policies and believes that Britain should be ruled by its own elected parliament while enjoying free trade with EU nations.
The party scooped 7% of the vote in the 1999 European elections and gained three seats.
But its finest hour to date came in 2004 when it won 16% of the vote in that year's European elections, beating the Liberal Democrats into fourth place.
The party gained a record 12 MEPs - making it a real force in the Strasbourg Parliament and boosting its profile considerably in the UK.
It was the sort of political breakthrough UKIP had been hoping for since its formation but it was followed by internal divisions as its star candidate, TV presenter Robert Kilroy-Silk, clashed with leader Roger Knapman.
Mr Kilroy-Silk said he wanted to "destroy" the Conservative Party and "wreck" the European Parliament but after failing to wrest control of the party he quit in frustration.
The former Labour MP then formed his own party, Veritas, Latin for truth, but quit that after disappointing general election results.
From the outset UKIP has been torn by internal divisions.
Party founder Alan Sked quit before the 1999 elections after arguing the party should refuse seats in the "gravy train" of the Strasbourg Parliament.
Mr Kilroy-Silk founded - and then quit - his own party
Shortly after that election the party's national executive lost a no confidence vote.
Leader Michael Holmes resigned in the ensuing debacle, though he remained an MEP.
His replacement Jeffrey Titford, also an MEP, narrowly beat academic Rodney Atkinson who quit, accusing the party of being "infiltrated by extremists".
Mr Knapman then took over the role of party leader in 2002 and led it to its greatest triumph at the 2004 European elections.
The party did not do as well as it had hoped at last year's election either, losing its deposit in more than 450 seats, and coming a long way short of the 16% of the vote it gained in the 2004 Euro elections.
Now the party has begun to establish itself as a force at Westminster by-elections, beating the Conservatives into fourth place in Hartlepool last year and, more recently, coming third, ahead of Labour, in Bromley and Chislehurst.
UKIP has also tried to capitalise on David Cameron's election as Conservative leader, hoping to attract a fresh wave of support from Tories disgruntled at the direction their party is going.
It has tried to broaden its policy agenda into areas abandoned by Mr Cameron - such as immigration and tax cuts - in his move towards the centre ground.
And its leaders have become embroiled in an increasingly bitter feud with Mr Cameron, after the Tory leader branded UKIP "fruit cakes, loonies and closet racists mostly".
UKIP threatened legal action over the accusation and has gone out of its way ever since to stress its anti-racist stance.
Mr Knapman, meanwhile, announced he was standing down after his four year term as UKIP leader, paving the way for a contest.