Public criticism of Labour leader Ed Miliband by party colleagues creates "an unappealing sense of toxic disunity", Tessa Jowell has warned.
The former Labour minister wrote in the Observer that people "who should know better" had helped the Tories create "a so-called summer crisis" for Labour.
Such "disloyalty" risked handing the next election to the Tories, she said.
Last week, Lord Prescott said the party had "massively failed" to get its case across to the public this summer.
Writing in the Sunday Mirror, he said Labour had also failed to hold the Tories to account, describing it as "a wasted opportunity".
And he urged Mr Miliband to "kick out" under-performing shadow cabinet members.
Those comments follow criticism of Mr Miliband from other Labour figures including shadow health secretary Andy Burnham who told the Guardian the party must "shout louder" and put its "cards on the table" by delivering attention-grabbing policies by the spring.
In the Observer on Sunday, Dame Tessa wrote that there were "complementary rights and obligations when it comes to the leadership of the Labour party: anyone may stand for the leadership but, once the winner is chosen, he or she is entitled to the loyalty and support of the party at every level".
"Loyalty is what keeps the boat afloat; disloyalty the rock against which it breaks."
The MP for Dulwich and West Norwood said there was "nothing constructive in publicly delivering 'helpful advice' which could be much better delivered quietly in private".
"For the public, it creates an unappealing sense of toxic disunity."
She said Labour politicians were "not commentators on a Westminster game of who is up and down".
"We are, rather, participants in a political contest whose outcome will affect the lives of millions of people.
"It is not the political class but our constituents who will pay the price if we allow David Cameron and the Conservatives another term in office - to squeeze living standards as prices rise faster than wages, to abandon families with elderly relatives and children waiting on trolleys in hospitals, or to take no responsibility towards those of our young people who are without jobs or hope of a home of their own."
Dame Tessa said her advice "to Ed's summer critics is to spend more hours on the doorsteps of our marginal seats busting a gut to help our candidates win - that is the real frontline".
Speaking on BBC Radio 5 live on Sunday, Mr Burnham said his earlier comments were "self criticism" and he was in agreement with Dame Tessa.
"It's people like me in the shadow cabinet - we need to be out there, week after week, exposing what this government is doing, but then putting forward positive plans that people can vote for," Mr Burnham said.
"My message is get behind Ed Miliband, we can win that next election."
Mr Miliband is widely expected to reshuffle his top team in coming weeks.
He has faced calls from some in his party to bring back familiar faces from the Blair/Brown era to beef up the party's media profile.
But last week, former home secretary David Blunkett told BBC Radio 4's Today programme it was clear "the oldies wouldn't be coming back so we need to find new ways of being able to contribute".
"What we could do better is probably us joining up with younger, enthusiastic, energetic, up-and-coming people so that we can give them a bit of advice - if they are prepared to listen to it," he said.