Labour will not win the next general election outright and will have to go into coalition with the Liberal Democrats, former Tony Blair spin doctor Alastair Campbell predicted.
The ex-Downing Street communications chief has been brought in by Ed Miliband to advise on the party's 2015 campaign.
But he said the party leader had failed so far to follow up his “one nation” approach with sufficient “hard-hitting policy” and predicted he would fail to secure an overall majority.
“I think it will be Lab-Lib coalition,” he told the left-leaning Fabian Review.
“I was right in 2010, I knew it was going to be a Tory-Liberal coalition.”
Mr Campbell said the Labour pledge to freeze energy bills for 20 months was “a good one nation policy” but suggested more was required.
“One Nation is not established. It means to the public whatever the public wants it to mean”, he said.
He added: “One Nation Labour worked last year for Ed's speech but it wasn't followed through with hard-hitting policy.”
There needed to be a “sustained campaign” behind other attractive initiatives, he suggested - “one on the economy, a couple on public services and one about cultural space”.
He told the magazine that rail re-nationalisation was a policy “people can latch onto, and should be explored further.”
Speculation over the possibility of a Lab/Lib deal has been reignited in recent days by shadow chancellor Ed Balls apparently extending an olive branch to Nick Clegg.
Mr Balls signalled that he could work with Nick Clegg in coalition after the next general election despite previously appearing to rule it out.
Conservative Party Chairman Grant Shapps said: “Even one of Miliband's most loyal henchmen realises Labour has no long-term plan for the country.
“They can only offer more of the same. More spending, more borrowing , more taxes and no amount of gimmicks will hide that.”
Mr Campbell's gloomy analysis of Labour's election hopes came as it was announced he had been recruited by GQ magazine to undertake “heavy-hitting interviews”.