Mr Gray conceded that Scottish voters might want an independence referendum but that the middle of a recession was the wrong time.

During a highly personalised attack on First Minister Alex Salmond, as he accused the SNP of being a campaign, not a government, he conceded that the nationalist keyplank policy – an Independence referendum – had merit.

“The day may well come when the people of Scotland want a referendum to settle their constitutional future once and for all. But not now, in the midst of a recession. And not on a question rigged and fixed by the SNP,” Mr Gray told conference.

Afterwards Mr Gray sought to clarify his remarks, insisting that there would be no change of policy this side of an election and that now was the wrong time for a referendum.

Less that two years ago Mr Gray backed the then leader Wendy Alexander’s surprise plan for an immediate referendum but during his own election campaign he dumped the policy. Yesterday he said he could not rule out a return to a pro-referendum policy.

“You cannot say in politics this question should never be asked,” he said. “Wendy said have a referendum now, and I supported her, on a straight question and Salmond ran way from that. (What) they are interested in is a rigged referendum and they are pursuing that in the middle of a recession when they should be focusing on getting Scotland through the recession.”

Asked if he would call a referendum if he were returned as First Minister Mr Gray said it would “depend on the circumstances at the time” and that the question should be a straight yes or no on independence.

The reference to a referendum “some time” distracted from Mr Gray’s attack on the SNP and his warning to Labour colleagues about what they risked if they lost power.

He accused the SNP of “cynically making promises they had no intention of keeping” and in his less personalised attacks said Mr Salmond had “no mandate, no majority and no shame”.

He shared the platform with Scottish Secretary Jim Murphy, who branded the current slate of Tory candidates as “the most hard-line in living memory”.

Willie Bain, Labour’s Glasgow North East candidate, also spoke to the conference, promising to stand up for Glasgow while he accused the SNP of “axing more of Glasgow’s services”.

Nicola Sturgeon, the SNP deputy leader, said Mr Gray had simply added another layer to his confused position on a referendum.

“On St Andrew’s Day, the Scottish Government will be publishing a white paper on Scotland’s future. The SNP government’s position is very clear,” she said. “We will bring forward the Referendum Bill next year. If Iain Gray wants a debate, he has a democratic obligation to support the bill so that the people of Scotland have the opportunity to choose their future.”