The Mail on Sunday says that Ed Miliband says that he would look at introducing border controls if he became prime minister of the rest of the UK after Scottish independence.
The Labour Press Team denied this story on Twitter, but Reuters reported Miliband as saying something similar back in June.
Back at the start of the year I wrote that purely negative arguments would not be enough for the No side to win the referendum:
were I Scottish, if anything could convince me to vote for independence it would be being told that I could not afford it. I would be strongly tempted to vote Yes just to spite such a foolish argument.And this looks likes a more negative argument than most.
But what if Ed Miliband is not threatening anyone? What if he is giving a sober prediction of the way things might well turn out after Scottish independence?
It easy to imagine that an independent Scotland would have more liberal immigration policies than the rest of the UK.
This is not because the Scots are inherently more left wing than the rest of us - I am sceptical of such arguments - but because its population profile would give it a greater need for young workers.
In those circumstances, it is quite possible that the rest of the UK would look at ways of stopping people coming to Scotland and then moving south.
When I suggested this on Twitter last night, some people pointed out that there are few border formalities between Northern Ireland and the Republic or even on the ferry from the Republic to Wales.
The border between Scotland and the rest of the UK might turn out to be like that. I hope it would.
But I suspect these informal arrangements are already under pressure, and Scottish independence would inevitable lead to a reconsideration of the relationships between the nations in these islands that would put them under more.
Today's reaction to the prospect of border formalities, along with my debate on Twitter last night, has confirmed to me that there are two odd things about the Scottish nationalist case.
First, those who argue for an independent Scotland are trying their best to prove that it will make no difference. We are told that Scotland will keep the Queen, keep the pound and have open borders with the rest of the UK. That seems a strange basis for demanding such an upheaval.
Second, while the political culture of the United Kingdom is so debased that Scotland must break free of it, once it does the rest of the United Kingdom will act with perfect fairness, even charity, towards it.
The truth is that, once Scotland is independent, the rest of the UK, though it will want to be a good neighbour, will seek to arrange relations between the two nations so they are as advantageous to the rest of the UK as possible.
And if the Scots thinks this sounds as though they will be pushed about a bit, they had better get used to it. That is what tends to happen to small nations.