Labour must be “credible” and “electable” in order to win back power and realise its “high ideals”, former PM Gordon Brown has said.
Mr Brown said Labour must not become “a party of permanent protest”.
He said he was not attacking a leadership candidate, but some comments appeared to warn against Jeremy Corbyn. Mr Corbyn’s team said he was most likely to engage non-Labour voters.
The other candidates are Yvette Cooper, Liz Kendall and Andy Burnham.
Mr Brown’s comments come as a poll suggested Mr Corbyn is considered the candidate most likely to worsen Labour’s prospects of winning the next election.
Polls also suggest Mr Corbyn is the front-runner in the leadership race.
Mr Brown did not use Mr Corbyn’s name but made it clear he disagreed with many of his economic and social policies.
He said the UK must continue to build international alliances under a future Labour government.
He added: “I have to say that if our global alliances are going to be alliances with Hezbollah and Hamas and Hugo Chavez’s Venezuela and Vladimir Putin’s Russia, there is absolutely no chance of building a world-wide alliance that can deal with poverty and inequality and climate change and financial instability, and we’ve got to face up to that fact.”
Mr Corbyn has previously described Hezbollah as “friends” and said that he wanted Hamas to be “part of the debate”.
Analysis: Alex Forsyth, political correspondent, BBC News
Gordon Brown didn’t need to mention Jeremy Corbyn’s name for his message to be understood; if he wins the leadership race Labour risks electoral oblivion.
The warning was couched in more subtle terms than those used previously by people who’ve called Mr Corbyn a potential car crash, but it was no less potent.
Steeped in references to Labour’s past, this was a speech designed to tug heart strings from a man who commands the attention of some on Labour’s left.
The question will be whether it makes any difference.
Many of Mr Corbyn’s supporters say he provides an alternative to the current political system – so they may not be swayed by the views of established politicians.
Mr Brown also referred repeatedly to the need for Labour to become more than a party of protest – while also warning it needed to be different from the Tories.
“What I am here to say is that the best way of realising our high ideals is to show that we have an alternative in government that is credible, that is radical, and is electable – is neither a pale imitation of what the Tories offer nor is it the route to being a party of permanent protest, rather than a party of government,” he said.
BBC political correspondent Ross Hawkins said part of the speech was as much a “coded dig” at Ms Kendall as it was at Mr Corbyn.
Mr Brown went on: “It is not a mistake to want power. It is not a mistake to do what is necessary to get back to power.”
The former MP also said the party had to offer “hope” to the public that it could be an alternative government.
“I believe that our vote is both a public duty and a sacred trust.
“It is a public duty because we have got to show that the Labour Party can be at the service of the country and that we can change society for the better in the future,” he said.
Last week, Mr Brown’s predecessor at Number 10, Tony Blair, warned that Labour risked annihilation if it elected Mr Corbyn.
A spokesman for Mr Corbyn said he was the candidate “most likely to engage with voters beyond Labour’s existing supporters” and credibility did not mean signing up to austerity.
Ms Cooper said it was “no good just being angry at the world if you can’t change the world”.
Mr Burnham said the country “needs a strong opposition” and Mr Brown and others “need to be listened to”.
Earlier, Ms Kendall insisted she had no intention of stepping aside in the race, with polls suggesting she is lagging behind other candidates.
Mr Brown spoke after a ComRes study of 2,035 adults in Britain, for the Independent on Sunday and Sunday Mirror, found 31% of those polled thought Mr Corbyn would worsen Labour’s prospects of electoral success. Mr Burnham was the poll’s least damaging candidate.
Conversely though, 21% thought Mr Corbyn would be most likely to boost Labour’s chances of winning the next election.
Labour leadership contest
- Who are the candidates? Andy Burnham, Yvette Cooper, Jeremy Corbyn, Liz Kendall
- Dates: Ballot papers were sent out on 14 August; voting can take place by post or online. They must be returned by 10 September. The result is on 12 September
- Who can vote? All party members, registered supporters and affiliated supporters – including those joining via a union
- What is the voting system? The Alternative Vote system is being used so voters are asked to rank candidates in order of preference
- How does it work? If no candidate gets 50% of all votes cast, the candidate in fourth place is eliminated. Their second preference votes are then redistributed among the remaining three. If there is still no winner, the third place candidate is eliminated with their second preferences (or third in the case of votes transferred from the fourth place candidates) redistributed. It is then a head-to-head between the last two candidates
Labour leadership poll
total electorate, though this may fall as party removes those not entitled to vote
Of which, full party members: 299,755
Affiliated to a trade union: 189,703
Registered to vote by paying £3: 121,295