Labour MPs are being insulted, trolled and threatened with deselection for expressing their views on issues of security and conscience, according to Chuka Umunna, the former shadow business secretary.

He called for the threats to stop and for the party once again to be a broad church. He also urged Jeremy Corbyn to give Labour MPs a free vote on the extension of RAF bombing to Syria, saying it was a matter of conscience and citing support from the shadow chancellor, John McDonnell, for a free vote.

He also said remarks by Ken Livingstone claiming the shadow defence minister, Kevan Jones, needed psychiatric help meant it was inappropriate for him co-chair the party’s defence review alongside the shadow defence secretary, Maria Eagle. Corbyn’s office did not inform Eagle that Livingstone was being appointed to the post, leaving her to discover the news on social media.

Livingstone reluctantly but unreservedly apologised for his remarks, after being forced to do so by the Labour leader.

The latest outburst of infighting reflects growing tensions as Corbyn tries to take a grip of the party machine, and considers imposing a whip to oppose an extension of airstrikes if there is no UN authorisation.

The internal dispute on Syria may be defused somewhat if France manages to win UN backing for all means necessary to be used to defeat Isis.

Speaking on BBC Radio 4’s Today programme, Umunna warned that the British public would not support politicians who failed to keep them safe. “Each individual MP has a mandate from those who elected them. We are primarily representatives of our constituents. The first duty of any elected representative, not just ministers, is to do all we can to ensure the security of our constituents, particularly in the face of the terrorist threat we are facing,” he said.

“This goes above and beyond party politics and, dare I say it, internal party politics. Because if you cannot keep the people safe, in their eyes that is a disqualification from office. The easy thing for many MPs would be to say: ‘I go along with every single comment, every single thing I have heard from the leadership’.”

Umunna continued: “Ultimately, there are people in our party who are pacifists, who would never ever sanction the use of lethal force or military intervention in any circumstances. I don’t share those views, nor do the majority of people in the country. But somehow we have to resolve these differences in a comradely way.

“The reality is, on some issues you agree that you cannot always agree. That is why I think it is absolutely fundamental that we have a free vote when that comes.”

Umunna said he had not decided whether to support airstrikes, but there was a growing mood in the party to back them, with many shadow cabinet members willing to defy Corbyn over the issue.

He said: “I have no fixed view on this, I have an open mind … The problem is, ultimately, I have a point of view which I have by reference to my principles and my values and my desire to keep the people that I represent safe. I am sorry, but that will come before any internal party political issue. And I think I should be able to adopt that position without being attacked, without being subject to a nasty troll form of politics.”

Lord Reid of Cardowan, a former home secretary, described the state of the party as sad for the country, saying Labour was not a coherent or competent opposition, with shadow cabinet members being forced to clarify the leader’s remarks.

He said it was for the current generation of MPs to decide how much longer they could tolerate the chaos and to fight the ideological battle with the left. He said: “It does not look coherent when the leader says something and, within a day, a shadow cabinet member and a series of them have to come out and disagree, contradict, clarify or say he was not saying what he was saying. I regret that, above all, from the point of view of the country.

“People like Brown, Blair and Blunkett – we did not have that victory fall into our lap. It was only after 15 years of political argument, and the defeat of the Bennites, the Scargillites the militant and the other entryists – all of who had they won would have taken us into the political wilderness.”