David Miliband has warned that the Labour Party risks transforming into a sect if it does not stamp out issues such as anti-Semitism and deselecting MPs.
The former Labour MP, speaking on Peston, said that people should be "sick" at the anti-Semitic abuse politicians such as Luciana Berger has received.
Miliband, who unsuccessfully ran for the Labour leadership, argued that current leader Jeremy Corbyn had a responsibility to ensure the party remained a coalition - and warned against it fragmenting.
He endorsed deputy leader Tom Watson's reaction to eight Labour MPs quitting to join the Independent Group, in which he said that it raised fundamental questions about the party.
"He [Watson] said they were good people whose departure was a source of profound sadness which raised fundamental questions for the Labour Party," Miliband told Peston.
"Above all, and this is my words, whether the Labour Party is going to remain a coalition or whether it's going to become a sect."
Miliband told Peston that Ms Berger had been through the "most appalling abuse and sectarianism".
He said that the Labour Party had been a great force of change for 100 years, bringing together the "broadest coalition" on the centre left of politics.
Asked if he thought Mr Corbyn was trying to turn Labour into a sect, Miliband said: "I think that certainly some of his supporters and the way they have demonised anyone who disagrees with them is utterly alien to the way in which the Labour Party has found success, not just for itself, but for the country over the last 100 years.
"I think there are responsibilities on Jeremy Corbyn and they need to be fulfilled.
"Because the truth is, against a government that by any standards is failing in its most basic responsibilities towards the country to steer it through this momentous Brexit debate... it is still at least equal and probably ahead of the Labour Party in the polls and that is a huge warning to all of us who really worry about the direction of the country."
Asked whether he thought that it was the right time for a new centre party, such as the Independent Group, to step forward, Miliband said: "I don't think so, because it remains the case that Labour is the home for the vast majority of people who have got opinions that I share and that I would describe as being on the centre left of British politics."