Former British prime minister Tony Blair appealed to members of his Labour party to prevent a left-wing socialist candidate being elected its new leader, warning of electoral "annihilation".
The bearded Jeremy Corbyn has ridden a surge of support from young people and new Labour supporters to move from being a rank outsider to the frontrunner to succeed Ed Miliband, who resigned after losing May's general election to Prime Minister David Cameron's Conservatives.
Blair, who was in power from 1997 to 2007 and is remembered for leading the party to landslide victories although his legacy was soured by the unpopular Iraq war, said his party was at risk of "self-destruction" if the anti-austerity Corbyn side won.
"The Labour party is in danger more mortal today than at any point in the over 100 years of its existence," Blair wrote in a fiery article in the Guardian.
"It doesn't matter whether you're on the left, right or centre of the party, whether you used to support me or hate me. But please understand the danger we are in."
"If Jeremy Corbyn becomes leader ... It will mean rout, possibly annihilation."
Corbyn, 66, is a veteran campaigner who has repeatedly voted against the Labour party line over his 32 years in parliament, supports scrapping nuclear weapons, and has never held a frontline ministerial position.
Seen as the furthest left of the four candidates in the leadership, the softly-spoken former union official has packed out a series of speaking venues, taking many commentators by surprise and prompting newspapers to declare an outbreak of "Corbynmania".
But more centrist Labour lawmakers and voters have expressed fear his policies are out of sync with the wider British electorate.
As the deadline passed for registering to vote in the election, several Labour lawmakers spoke out to express fears that infiltration by far-left or mischievous non-Labour members could twist the outcome of the race.
Labour has so far rejected applications from around 1,200 members or supporters of other parties who sought to join the list of registered supporters - at a cost of 3 ($5/four euros) - which would have allowed them to participate in the ballot.
Simon Danczuk, a high-profile Labour lawmaker who has called Corbyn's policies "crazy", called for the contest to be re-run.
"Hearing the horror stories from around the country in terms of entryism... I do think we're moving to a position where the election probably isn't tenable," he told LBC radio.
Another, Graham Stringer, said the race should be paused.
"People who basically have lied when they say they are Labour Party supporters are going to vote in the election," he said on LBC.
Around 440,000 people are eligible to vote in the election, which gets under way on August 14.
Labour says it has "no plans" to suspend or halt the race, whose results are due on September 12.
