Former British Prime Minister Tony Blair says his party faces "annihilation" should Labour elect far-left lawmaker Jeremy Corbyn as its leader.

Writing in the Guardian on Thursday, Blair urged the party to reject Corbyn, who leads opinion polls and has alarmed centrists with calls to renationalize some industries and withdraw from NATO.

Blair says the contest is no longer just about choosing the next leader — it's about whether Labour will remain a viable party of government.

"If Jeremy Corbyn becomes leader it won't be a defeat like 1983 or 2015 at the next election," Blair wrote. "It will mean rout, possibly annihilation."

Acknowledging that he is a divisive figure, Blair says Labour supporters should reject Corbyn "whether you used to support me or hate me."