Alex Salmond has challenged Nicola Sturgeon to reveal how she will "bend Westminster to the will of the Scottish Parliament" and persuade Boris Johnson to allow another independence referendum.
The former First Minister spoke out as the SNP leader prepared to launch the first of a new series of papers designed to form an updated prospectus for independence.
But the Alba Party leader said that without detail of how the UK Government could be made to change its mind and give the green light to another referendum, any debate about Scotland's future would take place in a "constitutional vacuum".
The question of how another referendum could be held without Westminster support is crucial as the Scottish Government prepares for a second vote - which Sturgeon has said will take place before the end of 2023.
Salmond stood down as First Minister in 2014 in the wake of Scots voting to stay part of the UK - when then Prime Minister David Cameron had agreed to the ballot taking place.
Since then Salmond's relationship with Sturgeon has soured, and he has gone on to set up his own rival pro-independence Alba Party.
Speaking ahead of the First Minister publishing the new paper - described as a "scene setter" - on Tuesday, Salmond said: "Everyone across the Yes movement is galvanised by the fact that the starting gun has been fired on another independence test.
"It is welcome to see Government documents proclaiming the case for independence but it is also crucial that the SNP/Green coalition articulate a clear pathway and timetable towards that day of decision."
He argued that "the energy of the independence debate of 2014 came from the setting of the date of September 18. The SNP/Green coalition have promised another referendum in 2023 with 'no ifs or buts'.
"The Yes movement will take them at their word, and they therefore should also explain the strategy by which they intend to bend Westminster to the will of the Scottish Parliament to agree a referendum or indeed spell out another way of asserting the sovereignty of the Scottish people.
"Without that we run the risk of conducting a debate in a constitutional vacuum."
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