A nonsensical scare story, insist the SNP. A genuine concern, say the pro-UK parties, if an independent Scottish Government were to pursue a radically different immigration policy from that of the United Kingdom.
Q: Would border posts be set up between an independent Scotland and the rest of the UK?
A: The Scottish Government is adamant that this would not happen. SNP ministers have repeatedly dismissed the suggestion as scaremongering on the part of the No campaign. The UK Government says there can be no guarantee, however. It says passport checks could only be avoided if an independent Scotland became part of the UK and Ireland Common Travel Area (CTA) and pursued similar immigration policies.
Q: Isn't that what the Scottish Government proposes?
A: The SNP says an independent Scotland would join the CTA and the UK accepts it would be possible. Most experts believe an independent Scotland would be able to opt-out from the Schengen area, which allows free travel between most European countries, during negotiations to join the EU. The real question mark is over the SNP's immigration policy.
Q: What does the Scottish Government propose?
A: Control over immigration would be one of the "major gains from independence," according to the Scottish Government. Alex Salmond has set a target to increase net annual migration to 24,000 as part of a package of measures to grow the economy and raise tax revenues by £5 billion by 2030. It would be achieved using incentives, such as a lower savings threshold for people wishing to move to Scotland, and allowing more foreign students to stay and work. Mr Salmond describes the rise as "modest" and points out the total is only 2000 higher than the average net inward migration to Scotland of 22,000 over the past 10 years.
Q: Would that level of increased immigration cause the UK Government to set up border posts?
A: Pro-UK politicians have been deliberately vague. Scottish Secretary Alistair Carmichael said an independent Scotland could not join the Common Travel Area if it pursued a "wildly diverging" immigration policy. Ed Balls, Labour's shadow chancellor, said border controls would be "inevitable" if Mr Salmond "makes a virtue of changing immigration policy".
Q: So it's not clear?
A: Reading between the lines, it seems unlikely that border posts would go up at Gretna if Scots vote Yes. In its independence blueprint the Scottish Government says it there are "no circumstances" in which it would do anything to prevent people from the rest of the UK travelling freely into Scotland. The UK Home Office, in its analysis of the impact of independence on borders, says only that an independent Scotland would have to "align certain visa and immigration policies" to become a member of the Common Travel Area.
Q: And what would happen to passports?
A: The Scottish Government says that all Scottish citizens would have the right to a Scottish passport, which would cost the same as and "broadly follow" the look of a United Kingdom passport. The United Kingdom Government says that it is "likely" that Scots would hold dual British and independent Scottish nationality, though the continued right to a British passport and other benefits of citizenship would be up for negotiation following a Yes vote.
UK Government paper, Scotland Analysis, Borders and Citizenship: "It is not possible to collaborate with other states in a borderless travel arrangement and at the same time to have an immigration policy that differs significantly from, or undermines the policies of, the other members. It is therefore difficult to see how an independent Scottish state could implement an entirely independent migration policy and ... be accepted as a member of a borderless travel zone."
Scottish Government White Paper on independence, Scotland's Future: "Within the Common Travel Area, an independent Scotland will work with the UK and Irish governments to ensure visa and immigration controls and practice meet certain shared standards. The detail of this would require negotiation but full harmonisation is not required; Ireland and the UK operate different immigration systems."
Scottish Secretary Alistair Carmichael: "They (the SNP) say we can be part of the Common Travel Area, which is an open border between Scotland and the continuing United Kingdom, and at the same time they say they will have a widely divergent immigration policy. I think it's pretty well accepted, and pretty well self-evident, that in fact you can have either of these things but you cannot have them both."
Deputy First Minister Nicola Sturgeon: "It completely ignores the reality of the Common Travel Area, in which the UK and Ireland already have no border controls but differing immigration policies.
"The only people wanting to erect borders are Mr Carmichael's own colleagues in the Westminster Government, who are threatening to drag Scotland and the UK out of the EU and the single market.
"It's time for less 'Project Fear' and more common sense."
Ed Balls, Labour's Shadow Chancellor: "If Mr Salmond makes a virtue of changing immigration policy, I think at the moment the view taken in the rest of the United Kingdom is that if a different country chooses to go down that road, then there would have to be checks on mobility across the Border.
"I think that's inevitable."
First Minister Alex Salmond, writing in The Herald: "The main Westminster parties have decided to deploy immigration as a weapon in their increasingly tawdry self-styled 'Project Fear' campaign.
"The United Kingdom Government and Labour Party are using an estimate that net annual migration needs to rise to 24,000 to match or exceed the same ratio of working people to pensioners in the UK as if it were something to be frightened of - a reason to vote No."
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