The UK Government has been urged to pass legislation setting out what extra powers the Scottish Parliament will receive in the event of a No vote.
Former SNP deputy leader Jim Sillars said the three unionist parties had all promised support for further devolution and MPs should provide voters with the information before they go to the polls on September 18.
The former MP challenged Prime Minister David Cameron, his predecessor Gordon Brown and Better Together campaign leader Alistair Darling to “put up or shut up”.
Mr Sillars, who was married to the late independent MSP Margo MacDonald, will publicly issue the challenge at a public meeting in Paisley tonight.
He will say: “The Better Together campaign has a golden window of opportunity – common purpose, a slack parliamentary timetable and a thirst in Scotland for detail.
“Let them act now – publish their promises, pass a Bill at Westminster, spell it out.
“A Bill could be tabled now, and acted upon with a second reading before the Westminster summer recess and our vote on September 18.”
Mr Sillars said the document could offer the detail many Scots say they want.
“Let’s have no more flannel from the ‘no’ side,” he said.
“Let Brown, Darling and Cameron stop writing cheques they can’t cash – put up or shut up.”