Former Labour leader Lord Kinnock has lashed out at "cowards" within the party over criticism of Ed Miliband's performance.
Miliband has dismissed the negative comments about his leadership as "noises off" but Lord Kinnock warned the party's MPs and peers they could damage Labour's poll chances because "in politics, division carries the death penalty".
Lord Kinnock, who endured bitter criticism both from political opponents and from within a deeply divided Labour Party during his time as leader, insisted that Miliband was not "disturbed or destabilised" by the attacks.
He claimed issues over whether Miliband was "ugly", "a geek" or if he committed "fratricide" against his elder brother and former leadership rival David were "entirely irrelevant".
Miliband has endured a difficult start to 2012, with Lord Glasman, the architect of the Blue Labour philosophy who has advised the leader in the past, complaining the party appeared to have "no strategy, no narrative and little energy" and former Cabinet minister Alan Johnson stating that Labour's "message is not getting through" to voters.
Lord Kinnock acknowledged that Miliband had to "draw more and more attention to huge achievements in the course of the last 14 or 15 months".
But in an interview with BBC Radio 4's The Week in Westminster Lord Kinnock aimed a fierce attack at "Mr Anonymous" - the off-the-record Labour critics of Miliband.
Lord Kinnock said the attacks on Mr Miliband were "in some ways much worse" than anything he experienced during his time at the head of the party "because it is less justified".
"Ed is highly intelligent. His intellectual strength is one of his greatest accomplishments. He is courageous," Lord Kinnock said.
He added: "If they are going to attack Ed, let it be on issues of substance, let it be on policy rather than on stupid things like: is he ugly, or is he a geek or did he commit fratricide - all of which are entirely irrelevant but constitute the main coverage of Ed Miliband."
Hitting out at those who briefed anonymously against the leader, Lord Kinnock said there were "gossips" and "self-serving fools" in all parties.
"First point, there are very, very few of them. Secondly, they lack the one thing that could give them significance - and that is guts. They are cowards."
He added: "In politics, division carries the death penalty and they must learn that because, in the end, it's their own chests they will stab as well as the back of the leader of the party."
Lord Kinnock said he had been hurt by personal attacks and been "merciless" in response, but Mr Miliband was "different" and "happily, less aggressive".
"I have no doubt he notices the attacks. But he is characteristically so cool and so calm that I am massively impressed by it.
"It is a characteristic of his that I knew a long time ago and is put into practice now in very heated and pressurised conditions.
"And he still - without in any sense being arrogant or complacent, quite the contrary - deals with it, manages it and keeps his focus firmly on the basic issues of concern rather than being disturbed or destabilised by these attacks.
"I think that's an admirable quality and certainly one you need in a leader."