Ukip leader Nigel Farage has said “all bets are off” for the 2015 General Election after his party gained its second directly-elected MP with victory over the Conservatives in the Rochester and Strood by-election.
Mark Reckless, who defected from the Tories to spark the showdown with his former party, won by a majority of 2,920.
The result is a serious blow for David Cameron, who personally spearheaded the effort to put a brake on Ukip’s recent surge with less than six months to go until the general election.
Mr Farage said the “massive, massive” win in what he called a “David v Goliath battle”.
It comes just weeks after another defector Douglas Carswell won in an identical contest in Clacton, Essex.
But the smaller-than-forecast margin of defeat may calm nerves that more eurosceptic backbenchers will be tempted to jump ship and join Nigel Farage’s party.
Labour - which suffered a polling-day embarrassment with the resignation of shadow attorney general Emily Thornberry over what was called a “snobbish” tweet about an English flag-draped house in the constituency - came third.
The owner of the house, Dan Ware, later said: “I’ve not got a clue who she is, but she’s a snob.” He said the flags had been left up since England played in the World Cup.
And the Liberal Democrats continued a humiliating string of by-election performances, finishing fifth with just 349 votes and losing another deposit.
A total of 40,113 votes were cast - a turnout of 50.67%.
In his acceptance speech, Mr Reckless his victory had proved that Ukip could win nationwide and urged voters to ensure enough MPs were elected to hold the balance of power after 2015.
“If you believe in freedom, if you believe in low taxes, if you believe in clean government, if you believe in localism, if you believe in people power.
“If you believe that the world is bigger than Europe, if you believe in an independent Britain, then come with us and we will give you back your country.”
The MP sought to put behind him a controversy over his apparent suggestion that existing EU migrants could be forced to leave the UK if it pulled out of Europe.
Mr Farage said the Ukip victory would put the result of the 2015 poll “up in the air” and said “all bets are off”. He told reporters: “(David Cameron) put his own personal reputation on the line to fight this by-election and lost, so there you go.”