Labour leadership hopefully Liz Kendall urges supporters to “back anyone” but rival Jeremy Corbyn.
Ms Kendall said Labour would be submitting its “resignation letter to the British people” if Jeremy Corbyn was to be elected as party leader.
She said the surprise frontrunner would stop Labour from being a serious party and raised questions about his commitment to keeping Britain in the European Union.
Mrs Kendall, who is polling in last place in the race for leadership, urged voters to use their second and third preferences, Andy Burnham or Yvette Cooper, in a bid to stop Mr Corbyn taking control of the party.
Speaking to BBC Radio 4, Mrs Kendall said: “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,” she continued.
Under the election voting system, if no contender wins 50 per cent of first preference, the bottom one drops and the second preferences of backers are relocated to second choice candidates.
Ms Kendall, who could finish fourth, is trying to sway supporters towards their second preference.
“I will be using my second and third preferences … It won’t be for Jeremy,” she said.
“I will be strongly urging all of those who are putting me as their first preferences to use their second and third preferences.”
She called for a voting pact with Ms Cooper and Mr Burnham, saying: “I have set out very clearly where I differ with all the candidates but our differences with Jeremy’s kind of politics are far greater.”
Mr Corbyn is set to launch his 10-point “Standing to Deliver” plan for Labour in Glasgow today 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.
He has promised a “new kind of politics” if he wins the leadership contest and his policies include a commitment to “growth not austerity”, nationalising the railways and energy sector, and a plan for nuclear disarmament.
Voters are set to receive their ballot papers today and the winner will be announced on September 12.