An independent Scotland could face the prospect of checks at its border with England, Home Secretary Theresa May has warned.
It comes after a conference speech when Ms May claimed independence could lead to mass immigration problems.
Afterwards, the home secretary said she envisaged "some sort of border check" if Scotland joined the European Schengen common travel area.
The SNP dismissed Ms May's comments as "scaremongering nonsense".
Ms May called for clarity on the issue as part of the independence debate.
The Scottish government has said it wants to hold the referendum on independence in autumn 2014.
At the Scottish Conservative party conference in Troon on Saturday, the home secretary highlighted the danger of joining Europe's Schengen area, which allows free movement of people within a number of EU countries.
She said: "Joining Europe's borderless Schengen area could open Scotland's border up to mass immigration."
In an interview with the BBC's Sunday Politics Scotland, Ms May added: "If there was a separate Scotland there could very well be a some sort of border check, but what that would be, to what extent that would be necessary, would depend on the issues about whether Scotland was in Schengen."
She added: "But the point I'm making is that these are exactly the sorts of issues that it is right to be discussing and should be part of the debate - should Scotland become separate."
SNP Home Office Affairs spokesman Pete Wishart said: "This is rank hypocrisy from Theresa May, who presided over one of the biggest immigration fiascos in recent memory, so Scotland needs no lessons from her on how to manage immigration.
"The reality is that an independent Scotland will be part of the common travel area which already exists within and between the UK and Ireland, so talk of border checks are nothing more than scaremongering Tory nonsense."