Labour MPs should put their own principles above party loyalties in the expected Commons vote on extending British airstrikes to Syria, former leadership contender Chuka Umunna has insisted.
Mr Umunna, who left the shadow Cabinet when Jeremy Corbyn took the top job, said he currently had an "open mind" on whether military action should be broadened.
But he argued that MPs should be granted a free vote when David Cameron brings forward proposals for dealing with Islamic State (IS) in the war-torn country.
Speaking on BBC Radio 4's Today programme, Mr 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," he said.
"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.
"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.
"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 Streatham MP said "very serious decisions" were going to have to be made over Syria, and pointed out that shadow chancellor John McDonnell had backed a free vote - although Mr Corbyn has since appeared to rule the option out.
"The reality is on some issues you agree that you cannot always agree," Mr Umunna said. "That is why I think it is absolutely fundamental that we have a free vote when that comes.
"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."
Mr Umunna also hit out at former London mayor Ken Livingstone, who recently responded to criticism over his role in a defence policy review by saying shadow defence minister Kevan Jones needed "psychiatric help".
"That is why I think Ken is completely inappropriate to be a co-convenor of our group looking at defence issues," Mr Umunna said.
Lord Reid, a Cabinet minister under Tony Blair, said Labour's response to the terrorist threat did not look "competent or coherent".
"I don't think their best friend would argue that we have been coherent on these issues," he told Today.
"It is sad not just from the point of the Labour Party, but the country. We need a competent, coherent opposition."
Asked whether it would be better if Labour MPs kept in line with Mr Corbyn, the peer replied: "If you mean that everyone else in the shadow cabinet said nothing and let Jeremy have his say, yes it would be coherent - whether it would be sensible or not, whether it would be appealing to the British people, whether it would be competent is an entirely different question.
"It doesn't look coherent if when the leader says something within a day a shadow cabinet person and a series of them have to come out, disagree, contradict, clarify or say he wasn't saying what he was saying.
"I regret that, it doesn't look competent."
Lord Reid said the question of how long the infighting would carry on was "a matter for the members of parliament, the new generation that has been elected".
"We didn't have that victory and that success in Labour fall into our lap," he said. "It was only at the end of 15 years of political argument and the defeat of the Bennites, the Scargillites, the Militants, the other entrants, all of whom had they won would have taken us into the political wilderness."