Mark Reckless was re-elected with 42 per cent of the vote in Rochester and Strood in southeast England, after defecting in September from Prime Minister David Cameron's Conservative Party to UKIP, which wants strict quotas on immigration.
Furious campaigning by the Conservatives failed to beat back the UKIP advance and their candidate lagged behind on 35 per cent, a blow to Cameron, who had vowed to "throw everything" at yesterday's by-election battle and visited the constituency himself five times to campaign.
"Whatever constituency you live in, whatever your former party allegiance, think about what it would mean to have a bloc of UKIP MPs at Westminster large enough to hold the balance of power," Reckless said.
"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."
UKIP leader Nigel Farage hailed a "huge, huge victory".
It is the second seat snatched by UKIP after another Conservative defector, Douglas Carswell, won UKIP its first elected seat in the national parliament in a September by-election in Clacton.
Speculation over further defections to UKIP swirled after Reckless suggested two more Conservative lawmakers could switch - an idea quickly dismissed by senior Conservative politicians.
Cameron has already promised a referendum on Britain's EU membership if his party wins next year's general election and has taken a harder stance on immigration in a bid to stem the flow towards UKIP.
"UKIP was not supposed to win this by-election," said Matthew Goodwin, politics professor at Nottingham University.
