The Labour party lost one of its safest parliamentary seats after a one-off vote in northern England, in the strongest sign yet that new leader Ed Miliband is failing to cash in on disenchantment with the Conservative government.
George Galloway, an anti-war campaigner in the small, left-wing Respect party, beat Labour's Imran Hussain in a result announced on Friday with more than 18,341 votes from a by-election on Thursday for the seat of Bradford West.
Galloway, who unsuccessfully ran for office in the 2010 general election and the 2011 Scottish Parliament elections, had a plurality of more than 10,000 votes.
He described the win, which toppled Labour from a seat it had held since 1974, as the most sensational result in British by-election history.
Labour has been hit by a tidal wave in a seat it held for many decades in a city it dominated for 100 years, Galloway said.
Labour candidate Hussain came second with 8,201 votes, while Conservative candidate Jackie Whiteley was third, with 2,746. Turnout was 50.8 percent.
Galloway, a divisive figure on the left, was expelled from Tony Blair's Labour party in 2003 for his vociferous opposition to the Iraq war - having accused Blair and George Bush of attacking Iraq like wolves.
(Additional reporting by Stephen Mangan; Editing by Michael Roddy)