Labour would be submitting its resignation letter to the British people if Jeremy Corbyn wins the leadership, his rival Liz Kendall has said.
Ms Kendall said the surprise frontrunner in the leadership contest would stop Labour being a serious party of government if he took power.
She raised questions about Mr Corbyn's commitment to keeping Britain in the European Union, claiming he would not rule out voting No in the upcoming in/out referendum.
Ms Kendall, who is trailing in the race, has urged voters to use their second and third preferences in the election to cast their ballots for either Andy Burnham or Yvette Cooper in a bid to stop Mr Corbyn taking control of the party on September 12.
Under the Labour election rules, if no contender wins outright with more than 50% of first preferences, the bottom one drops out and the second preferences of their backers are reallocated to the other candidates.
With the polls indicating Ms Kendall could finish fourth, her supporters' choice of second preference could influence the result.
'Millions of people depend on us'
The Leicester West MP told BBC Radio 4's Today programme: "He (Mr Corbyn) is not offering anything new, his programme isn't new, it's exactly the same as it was in the 1980s and we'll get the same result.
"I don't want to see Labour submit our resignation letter to the British people as a serious party of government.
"Millions of people depend on us to provide an alternative and that's what I'll be doing over the coming weeks."
Ms Kendall also raised concerns about Mr Corbyn's commitment to the EU.
She said: "I was at a hustings in Warrington where he was explicitly asked 'would you rule out voting No to Britain staying in the European Union?'
"And he said no he wouldn't rule it out."
Her comments echoed those of Ukip leader Nigel Farage, who highlighted Mr Corbyn's concerns about the EU.
Writing for Breitbart, Mr Farage said: "Whilst I have absolutely no faith in the wisdom of his economics, his victory is seriously good news for the No campaign in the forthcoming EU referendum.
"Under Corbyn, there will be a debate about TTIP (Transatlantic Trade and Investment Partnership), about the way Greece has been treated, and about the role of national parliaments and democracy.
"I have no idea which side of the fence Mr Corbyn himself will come down on, but I feel the Left is finally waking up to what a corporatist, big business club the EU really is.
"I have to say I hope he wins. The best news of all? A Corbyn win will be the death of the Green Party. Hooray."
Mr Corbyn is set to launch his 10-point Standing to Deliver plan for Labour in Glasgow and will post a copy to all party members in the hope of securing their vote as the 600,000 ballot papers start to arrive on doormats.
The new leadership rules have seen more than 120,000 people pay £3 to qualify for a vote alongside longstanding members - raising fears of political opponents undermining the process - and almost 190,000 from trade unions and other affiliates who have signed up.