This story was amended on Wednesday 13 April 2016 to remove the suggestion John Woodcock’s Twitter message had been intended for a journalist. We accept this was not the case and apologise for the error.

Labour must introduce “protocols and boundaries” around MPs who brief journalists against the leadership, Jon Cruddas and Stephen Kinnock have said.

Speaking at the launch of Consensus, a new group which says it aims to develop party unity, Cruddas said Labour MPs must stop using “wretched” meetings of the Parliamentary Labour Party (PLP) as an opportunity to damage the leadership in the eyes of the public.

“I was at a meeting of the PLP – though I don’t go to them much anymore because they’re so wretched in terms of the debate – someone got up and asked Jeremy Corbyn ‘will you be wearing a white poppy at Remembrance Sunday?’ I go out and the first question he’s asked is ‘will you be wearing a white poppy?’ I thought maybe there’s a game being played here”, he said.

“The use of PLP should be safe haven to work through some of these issues – but there’s a game being played with the press trying to take lumps out of the leader.

“There has to be boundaries to that sort of behaviour. It’s the end if you allow that culture to develop. You have to re-establish protocols and boundaries on these sorts of behaviour. You expect that of everyone who holds a Labour ticket.”

His comments were echoed by Stephen Kinnock, who advised the leadership to “keep calm and carry on”.

“It’s absolutely disgraceful that you see people texting things to journalists sitting outside of the room.

“Sometimes we’re sat in the PLP and someone will say the latest headline is based on something that was said three minutes ago in the PLP.

“The PLP should be a hermetically sealed place where you certainly do not tweet things to the media. That is absolutely unacceptable.

“The question is what we do about it. I think that is a very challenging thing for the leadership. If you act in certain ways you can make martyrs, and you may just add oxygen to the whole idea of the conflict.

“I think probably the thing to do is to ignore it – to keep calm and carry on. I don’t know what else you can do without shooting yourself in the foot. That’s my personal view of how to deal with the issue.”

This follows John Woodcock publicly tweeting that the leaked “hostility” list of MPs was a “f****** disaster”, which he later said was intended as a private message to a colleague.

Cruddas urged Labour members to continue searching for unity, to avoid the “danger that the centre empties out, becoming a death spiral within”.

“Don’t just become binary world of people who are taking lumps out of people. It’s life-denying and if we ignore the electorate the electorate ignore us.”

Consensus organiser Alex Chai said the group aimed to tackle the “bitter, self-defeating, counter-productive infighting” that had grown since last Summer.

The group will be hosting a seminar series aimed at developing an electorally compelling agenda for 2020. Over the next year Consensus will be hosting events with Clive Lewis, Stephen Kinnock, Richard Murphy, author of Corbynomics; Angela Eagle, and Professor Anastasia Nesvetailova, who advises John McDonnell as part of Labour’s economic advisory council. Each event will be focused on developing policy to unite the party as well as appeal to the electorate.

Many wings of the party were present at the launch, with Corbyn- supporters John McDonnell and Clive Lewis attending, along with “moderate” MPs Seema Malhotra and Stephen Kinnock, as well as many CLP members.