A fresh war of words has broken out over the prospect of passport controls in the wake of Scottish independence.
The UK Government warned action would have to be taken if SNP ministers pursued a “considerably different” immigration policy to other countries in the union.
It said that people would need a passport to cross the English border if Scotland was required to join the European borderless travel area, known as Schengen
A UK Government paper said the country would only be able to be part of a common travel area with the UK and Ireland if it “agreed to align some of its visa and immigration policies”.
But a Scotland Office spokesman said that prospect was “very difficult” to envisage because the SNP wanted this country to have a different immigration policy.
The warning came after SNP External Affairs Minister Humza Yousaf said the UK’s current immigration policies were “wrong” and people who choose to make an independent Scotland home “will always be welcomed”.
He claimed healthy population growth was “vital for economic growth” and a Yes vote would provide an opportunity to address the country’s demographic challenges.
Mr Yousaf said: “It would give Scotland the ability to tailor a robust new approach to migration to address our own specific social, economic, educational and demographic needs.”
The minister said an independent Scotland could bring back the post-study work visa system which was scrapped by the UK Government because it claimed the system was being “abused” by people using the route to go into low-skilled work.
Aberdeen and Robert Gordon (RGU) universities and Aberdeen College claimed the reforms had led to fewer postgraduate students from outside the European Union applying to study, and a drop in income.
A Scotland Office spokesman said: “It is simply not possible to collaborate with other states in a borderless travel area and at the same time have an immigration policy that differs significantly from the other states.
“The Scottish Government has said they want an independent Scotland to have a very different immigration policy from the UK. That makes it very difficult to see how a common travel area could be agreed between the UK and an independent Scotland.”
A Scottish Government spokeswoman said last night: “Following independence Scotland would remain part of the Common Travel Area, which has existed for many decades allowing freedom of movement between the UK, the Republic of Ireland, the Isle of Man and the Channel Islands.
“The Republic of Ireland and the UK already operate different immigration systems within the CTA, demonstrating that complete harmonisation of immigration policy is unnecessary.”