Firebrand British politician George Galloway, scourge of the mainstream parties, has won a sensational return to parliament.

Running for his Respect party, Mr Galloway swept to victory in the Bradford West by-election, dealing a heavy blow to his former party, Labour, which had held the seats for decades.

George Galloway testifies before the US congress in 2005. Credit:AP/Dennis Cook

Mr Galloway claimed 18,341 votes, more than 10,000 greater than Labour candidate Imran Hussain, confounding pre-poll expectations.

"This is the most sensational victory in British by-election history," the renegade socialist politician claimed in his victory speech.