The Scottish Liberal Democrat leader will warn the SNP not to try to “secure independence by the back-door”.
Willie Rennie is due to tell Lib Dem members that both unionists and nationalist parties face “big challenges” ahead as proposals are drawn up to deliver the extra powers for Scotland promised during the independence referendum.
On the instructions of Prime Minister David Cameron, Lord Smith is chairing a cross-party group to put concrete proposals forward which will form the basis of any future devolved powers.
At an event in Perth tonight, Mr Rennie will say that following the referendum, Britain and Scotland will never be the same again.
“The change we must secure needs to stand the test of time,” he will say.
“For nationalists, they will need to convince voters that they will not use the Smith process to secure independence by the back door only days after it was rejected by the voters.
“An attempt from nationalists to redefine home rule and federalism in an ultra-extreme form is perhaps understandable, but it is not something that will create a sustainable settlement that will stand the test of time.
“For unionists, especially Labour, the test will be whether they can adopt a more substantial package of powers that will give the parliament the financial responsibility and flexibility it needs to tackle inequality, create jobs and invest for the long term.
“Without the power to raise the majority of the money we spend, undertake prudent borrowing and control wealth taxes, the change will not be substantial enough to be sustainable.”