George Galloway has inflicted a crushing humiliation on Labour as he made a sensational political comeback in a by-election he termed the "Bradford Spring".
The controversial Respect politician won a stunning victory in the Bradford West by-election, defeating Labour by more than 10,000 votes.
The Conservatives were pushed into third place after the Budget backlash and fuel chaos.
Liberal Democrat candidate Jeanette Sunderland came fourth but polled so few votes the party lost its deposit.
But it was Labour and their leader Ed Miliband who were left stunned and perplexed by the scale of their defeat and Mr Galloway's victory.
Mr Galloway took over 55% of the vote share - double that of Labour, which lost around 20%.
In his victory speech, Mr Galloway invoked the series of uprisings that have toppled dictatorships across the Arab world as he called his victory the "Bradford Spring".
He told Sky News that the poll was "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," he said.
He said the people of Bradford had felt "neglected, even betrayed" by the "path of treason" set by Tony Blair in 1994.
And he blamed New Labour's involvement in "foreign war after foreign war" for leading the party astray.
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He was no less scathing about the Tory campaign, calling the by-election a "miserable, pathetic performance by the Government".
As a jubilant Mr Galloway left the sports hall he was mobbed by his supporters and had to retreat back into the building.
As he left again he was hoisted onto the shoulders of supporters as people chanted "Respect", while others said: "We love you George".
Mr Galloway was carried around the side of the building as his supporters continued to chant and cheer.
Bookmakers Ladbrokes said the result was the largest by-election loss in the company's history. They had suspended betting on Thursday afternoon after seeing a surge of support for the Respect candidate.
The by-election was caused by the resignation due to ill health of Labour MP Marsha Singh, who had held the seat since 1997 and had a majority of 5,763 over the Tories at the 2010 general election.
The result was declared at around 2.30am after a night of high drama and tension in the Richard Dunn Sports Centre, named after the city's former British boxing champion.
And after a short, three-week campaign in which he toured the constituency in a battlebus and wooed Asian voters with an uncompromising anti-war message, Mr Galloway delivered a knock-out blow on Labour.
Shortly after the polls closed at 10pm, senior Labour MPs were telling Sky News they were "confident, but not complacent".
But an hour or so later, the same Labour MPs were getting the jitters as it became clear that Mr Galloway was polling much better than they expected.
By midnight, the mood in the Labour camp was one of deep gloom as the party's MPs began to predict a win for Mr Galloway and MPs from other parties forecast a big victory margin over Labour.
At around 12.30am, Mr Galloway claimed victory on Twitter, declaring: "By the grace of God we have won the most sensational victory in British political history."
Labour MP Toby Perkins said the result was "desperately disappointing", but pinned Mr Galloway's success partly on his celebrity status from having appeared on a version of reality TV show Big Brother.
Mr Perkins conceded, however, that his party had lessons to learn from the way Mr Galloway had been able to "capture the mood" of the electorate, especially young people.
He went on: "I think frankly there wasn't a lot the other parties could do about it. They'd seen him on (Celebrity) Big Brother.
"They wanted him on their streets and now they've got it, and let's hope that he lives up to the promise that he's made to them and actually delivers on the optimism that surrounds his campaign, which hasn't always been the experience of constituents who have had George Galloway as their MP in the past."
"It must be a huge humiliation for Ed Miliband and his team," said Tory MP Kris Hopkins.
The Conservatives have not won in the area in 42 years and so did not have "high expectations", he said.
He added: "We probably have not had our best 10 days - a difficult Budget and a whole range of issues.
"But still, despite all that, Labour and Ed Miliband were not able to find any traction."
Mr Galloway, who was a Labour MP in Glasgow until his expulsion from the party over his opposition to the Iraq War, was making his third attempt at a comeback in the past two years.
After defeating Oona King in Bethnal Green and Bow in 2005, he failed in a bid to oust former Labour minister Jim Fitzpatrick in another east London seat, Poplar and Limehouse, in 2010 and then unsuccessfully stood for the Scottish Parliament last year.
In between, he earned a living as a radio presenter and appeared on Celebrity Big Brother on TV, dressing as a cat and crawling on all fours.
His aggressive, anti-war campaign in Bradford West, in which he personally targeted Imran Hussein, infuriated Labour campaign chiefs.
Mr Galloway directly appealed to Muslim voters, who make up nearly 40% of the electors in the constituency.
While the Tory high command will claim Bradford West is a safe Labour seat and the by-election was held mid-term, there will be alarm at the slump in the Conservative vote.
With vital local elections coming up on May 3 - including the high-profile clash between Boris Johnson and Ken Livingstone for London mayor - there will be concerns that the big drop in Tory support has been largely self-inflicted.
First came George Osborne's deeply unpopular Budget and the row over the "granny tax" and the 50p tax cut .
Then came the cash-for-access scandal in which donors apparently paid to dine with David Cameron at 10 Downing Street - and now the panic in government over the threatened fuel strikes and VAT on hot food .
The result was also bad for Nick Clegg and the Liberal Democrats, who will also fear a drubbing in the local elections.