The former chancellor, Alistair Darling, has blamed Labour's election defeat on the party's failure to talk openly about the UK's massive deficit.
During a lecture at the Edinburgh International Book Festival, Mr Darling said Labour was "sidetracked into a debate about investment over cuts".
Instead, he claimed the party should have been addressing "legitimate public concern over the level of borrowing".
Mr Darling was giving the Donald Dewar memorial lecture in the city.
The shadow chancellor, who is preparing to step down from Labour's front bench, told those at the event, in memory of the former Scottish first minister, that his party had "lost their way".
"By failing to talk openly about the deficit, and our tough plans to halve it within four years, we vacated the crucial space to make the case for the positive role government can play," he said.
"Because, unlike our opponents, we are clear that government has a crucial role in supporting growth - the prerequisite for getting borrowing down.
"But you will only convince people you've got the answers, if they believe you know what the question is in the first place.
"You can't have political credibility without economic credibility."
The Edinburgh South West MP warned that while Labour needed to criticise the coalition's spending curbs, it must "accept the need to cut the deficit".
The former chancellor also mounted a staunch defence of his handling of the credit crunch, insisting: "For the avoidance of doubt, I believe we handled the financial crisis of the past few years with competence, integrity and above all an absolute determination on my part not to put short-term political advantage before the interests of the country and, specifically, of risking a second period of recession."
But in a pointed reference to Mr Brown's mantra of "investment over cuts", he used the same phrase to illustrate how he believed the party lost the support of the electorate.
He said: "Labour lost because we failed to persuade the country that we had a plan for the future. What is important now for our party is we take stock and be honest about what went wrong."
He added: "You will only convince people you've got the answers if they believe you know what the question is in the first place. You can't have political credibility without economic credibility."
Mr Darling, who served as chancellor from 2007 until May 2010, also used the event to reaffirm his support for former foreign secretary David Miliband who is in contention to become the new Labour leader.
"I sat beside him in the cabinet and I know he is willing and able to make the tough decisions in the best interest of this country," said Mr Darling.
Mr Miliband is contesting the leadership with brother Ed, Ed Balls, Andy Burnham and Diane Abbot.