He said the party was running the “very dangerous” risk of “becoming a sect” after suggesting the party was becoming “tribal”.
But he insisted he remained “very committed” to the party despite Mr Corbyn’s leadership, which he said he did not vote for.
Mr Miliband told BBC Radio 5 Live: “Of course I always vote Labour, I argue for Labour, I’m still a member of the Labour party, I grieve for the state of the Labour party, that’s obviously the case, but I am of course very committed to Labour politics.
“I’d also say that we’ve got to recognise that one of the things that all political parties have to do is reach beyond their tribe.
David Miliband said he backed Owen Smith over Jeremy Corbyn for Labour leader
If political parties just become ‘tribal’ then they become a sect and that is very dangerous
“If political parties just become ‘tribal’ then they become a sect and that is very dangerous.”
He then admitted Labour was in “danger” of becoming a “sect” and squirmed at the latest polls putting support for Labour at 25 per cent – the lowest support level since 1983.
He told the presenter: “Please don’t rub it in.”
When asked if he would join Mr Corbyn if he was asked to be a senior member of his team, the former Foreign Secretary batted away the idea.
Jeremy Corbyn in pictures Tue, July 12, 2016
Labour Leader Jeremy Corbyn takes part in a community meeting at the Guru Har Rai Gurdwara Sahib temple
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Britain's opposition Labour Party leader Jeremy Corbyn speaks at an anti-racism rally in London
Ken Livingstone has been suspended from the party following his comments
He said: “I think that is the stuff of Jeremy Corbyn’s nightmares, it’s not the stuff of his dreams.”
He continued: “But the important thing and maybe this is something that is clearer to me now then it would have been 20 years ago.
“It is that the big thing in politics is not to define your own reality and then fit the facts to it, it’s to look at the world in a clear way and apply your values to it.”
The interview comes as controversy continues to surround the party after Ken Livingstone was spared expulsion but was suspended for his comments on Adolf Hitler and Zionism.
Mr Corbyn has since launched another probe into the former Mayor of London’s conduct.