One of Britain's biggest unions has backed left-wing contender Jeremy Corbyn to lead Britain's opposition Labour Party, and a major bookmaker has made the former outsider odds-on favorite to win the contest.

General secretary Dave Prentis of Unison, which represents more than 1 million public-sector workers, said the choice of Corbyn "shows a clear need for change towards a fairer society."

The endorsement doesn't compel union members to vote for Corbyn against rivals Andy Burnham, Yvette Cooper and Liz Kendall.

Corbyn's surge has alarmed senior Labour figures, who fear he is too extreme to win elections.

On Wednesday, bookmaker William Hill made Corbyn 11-8 favorite to win the contest, having ranked him a 200-1 outsider when the race started.

Voting opens Aug. 14, with the result announced Sept. 12.