Ed Miliband must work harder to get his message across to the public if Labour is to recover from a "shaky start" to 2012, senior Labour MP Alan Johnson warned.
The former cabinet minister said he believed the party leader had "the personality, the intellect and the steely determination to succeed".
But he said that unless he got out "on the stump" to gain public recognition and made the Opposition sound more relevant to ordinary voters it would be in trouble.
Johnson, who stepped down as shadow chancellor almost a year ago for personal reasons, wrote in The Mirror that it was a "pivotal year".
The party, he said, was "in good shape" - reserving particular praise for the most senior shadow cabinet members including his replacement Ed Balls.
"Yet the message is not getting through," he cautioned.
"Small opinion poll leads, welcome as they are, cannot hide the fact that the public remain suspicious about Labour."
That was not helped by "unfortunate comments by people who profess their support for the leader they are busily undermining", he said - in a swipe at Lord Glasman.
The Labour peer claimed last week that the leadership appeared to have "no strategy, no narrative and little energy" during a tough week for Miliband.
But Johnson conceded that the leadership was not helping itself by failing to present its policies effectively and too often sounding like a "debating society" or "university seminar".
"At the moment Labour is like a pearl stuck firmly inside an oyster. Inside there's a gem but the public can only see the shell," he wrote.
"In my view, Ed Miliband has the personality, the intellect and the steely determination to succeed - to set out a vision for a better society, to formulate a clear message and to face up to the difficult priorities that will need to be set."
But that required "a better grip on messaging, including over who speaks and when", he added, and recognition by shadow ministers that they had to offer a "credible alternative" as well as criticising the coalition.
"The leader must be the authentic voice and should be the public face of the Labour Party. That means going out on the stump, constantly identifying those issues which resonate with the public, and hammering home Labour's message," he concluded.
"We cannot get to 2015 and an election with the public and the media asking the question: Who is Ed Miliband?"
In the phrase repeated by allies over the weekend, Mr Miliband described himself as "someone of real steel and grit".
He is expected to use a speech about the economy tomorrow to try to seize back the political initiative.