George Galloway has warned of a "tidal wave" of disillusioned voters ready to rebel against all three mainstream parties around the country as he celebrated his shock victory in the Bradford West by-election with an open-top bus tour of his new constituency.
Addressing jubilant activists outside his campaign HQ, the Respect Party MP said Bradford had spoken for voters who want "political leaders they can believe in, who say what they mean, do what they say and don't lie to people".
Mr Galloway's victory on a remarkable 36.5% swing was a catastrophic result for Ed Miliband, who had not been expected to face a serious challenge in a safe seat at a time when Labour is riding high in the polls nationally.
The Labour leader promised to learn the lessons from the defeat, and said he would visit Bradford in the weeks to come to find out what went wrong.
"It was an incredibly disappointing result for Labour in Bradford West and I am determined that we learn lessons of what happened," said Mr Miliband.
Mr Miliband is facing tough electoral tests in the polls for local councils and the London mayoralty on May 3, and may also be confronted with a rash of parliamentary by-elections this autumn, with several MPs expected to quit Parliament to stand for police commissioner or elected mayor in cities including Bradford in November.
Mr Galloway said Labour had lost touch with its traditional core voters, telling BBC Radio 4's The World At One: "If Labour doesn't go back to being a Labour Party again, it will never command the support - let alone the enthusiasm - of the kind of people who supported it for at least 100 years."
All three main parties were facing a backlash from voters angry over austerity and British involvement in wars overseas, he said.
"There is a tidal wave waiting to break all over the country, not just in Bradford," said Mr Galloway. "There are very large numbers of people disenchanted and alienated from the political process and from all three major parties. If a backside could have three cheeks, then British politics is that three-cheeked backside."
Mr Galloway, an ex-Labour MP who was expelled from the party in 2003 after urging British troops to disobey "illegal" orders to invade Iraq, won 18,341 votes to the 8,201 for Labour candidate Imran Hussain in a contest sparked by the resignation due to ill-health of Marsha Singh.