Ex-Labour leader Ed Miliband has thrown his weight behind Owen Smith in his bid to be the party's next leader.
Mr Miliband said Mr Smith was the only candidate who can "rise to the challenge" of uniting Labour and taking the party back into power.
Mr Smith is challenging Jeremy Corbyn for the Labour leadership in a contest that will be decided in September.
Mr Miliband, who quit as Labour leader after losing the 2015 general election, said Jeremy Corbyn's postion was "untenable".
"For his vision, principles and ability to lead, my choice in this leadership election is Owen Smith - I hope he'll be your choice too," Mr Miliband said in a video message.
Jeremy Corbyn has previously described Mr Miliband as a "great friend" and he was consistently rumoured to be joining the current leader's shadow cabinet.
But Mr Miliband, who led the party before Mr Corbyn, said Mr Smith is the best candidate to make the crisis following the EU referendum "a progressive moment".
The leadership challenge has added fresh tension to an already fraught party which has seen many of the party's MPs withdraw support for Mr Corbyn in recent weeks.
However the leader is thought to retain the support of the majority of members.
Yesterday the party was also told it had wrongly barred tens of thousands of new Labour members from voting in the upcoming leadership challenge.