An independent Scotland could see checks introduced at its border with England, Home Secretary Theresa May has said.
May yesterday warned that independence could result in the country's borders being opened up to "mass immigration".
In an interview with BBC Scotland's Sunday Politics programme today, she said there could be a border check if Scotland joined the European Common Travel Area, by signing up to the Schengen Agreement.
She said: "If there was a separate Scotland, there could very well be 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, whether it wanted to be in the Common Travel Area which they would have to negotiate, if that was what they wished to do."
May said such issues needed to be addressed as part of the debate ahead of a referendum on independence, which the SNP wants to hold in the autumn of 2014.
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."
At the Scottish Conservative Party conference in Troon yesterday, Ms May warned that if an independent Scotland joined the European Union (EU), it "almost certainly" would not have the UK's opt-out on justice and home affairs matters, and would face joining Europe's Schengen area, which applies to all member states except the UK and Ireland.
The Schengen area operates border controls for the whole area, but there are no internal border checks between countries within the area.
A number of constitutional experts and unionist politicians have warned that Scotland would have to re-apply to Europe if it separated from the UK.
They argue that the rest of the UK would retain the opt-outs it has negotiated over the years, including the right to hold passport checks at the UK border through its opt-out of the Schengen Agreement, but Scotland would not.
Some have argued that this will result in the necessity for passport checks at the Scottish border but the SNP insists it will share the UK's successor status and retain its opt-outs.
The SNP said an independent Scotland would also inherit the Common Travel Area which exists across the UK and Ireland.
May said it could not be assumed that Scotland would be part of the Common Travel Area.
She added: "There is an assumption that it would be but we would need to be talking to Scotland about whether that would be right and about what that would entail."