Owen Smith will launch his bid to be Labour leader by warning the party risks “falling apart” if it does not act “like a proper team”.
The former shadow Work and Pensions Secretary will say his plan to invest £200bn into building projects is what is needed to “rebuild Britain”.
He joins Angela Eagle in the race against Jeremy Corbyn’s leadership.
Meanwhile Mr Corbyn has said he wants Labour’s governing body to change the rules for who can vote in the contest.
Writing in the Sunday Mirror Mr Smith said it was a “worrying time for Labour” and claimed the new Conservative government “will try to use Brexit as an excuse for slashing workers’ rights and forcing through even more cuts”.
“Only a strong, united and competent Labour Party can stop that happening and start leading us back to power. I am part of a new generation of Labour women and men who must now step up to secure Labour’s future,” he said.
The MP for Pontypridd will lay out his leadership pledges in a speech in his constituency later, having postponed his launch in the wake of the attacks in Nice.
He will talk about his multi-billion pound plan “to renew our crumbling buildings and transport systems and tackle our housing crisis, as well as investing in young people”.
He is also expected to attack Mr Corbyn for being anti-austerity without bringing forward an alternative.
“It is not enough for the Labour Party to simply talk about being anti-austerity. We have to set out a detailed plan for how Labour would replace failed Tory austerity with credible proposals for prosperity.”
Writing in the Observer, Ms Eagle echoed Mr Smith’s view that the party was in danger of being destroyed , saying it was “as divided and disunited as I can remember”.
She added: “Betrayal narratives do not unite; they divide and Labour will pay a huge price if this is allowed to continue. But it’s not too late to get our house in order and I know I can unite us again.”
Ms Eagle had hoped for a clear run at trying to defeat Mr Corbyn, the BBC’s political correspondent Ben Wright said.
However having announced his intention to stand, Mr Smith later added that he believed only one challenger should appear on the final ballot.
He suggested the party’s deputy leader, MPs or executive committee could choose between him and Angela Eagle.
Despite facing a revolt from his MPs, Mr Corbyn retains the strong support of many party members and has said he will fight the challenges in a contest which is expected to be decided in September.
Mr Corbyn, who will automatically be on the leadership ballot, has told the BBC’s Sunday Politics programme he thinks voting rules should be changed.
Last week the party’s National Executive Committee ruled that party members could only vote in the upcoming election if they joined before 12 January.
However, registered supporters who pay £25 before Wednesday will get a vote. Mr Corbyn believes the £25 fee is too high and wants Labour’s NEC to think again.
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