An independent Scotland would need to attract 20,000 more immigrants a year in order to pay for pensions without raising taxes, according to a think tank.
The Scotland Institute said the Yes campaign had significantly underestimated the number of new entrants required to balance out the country's ageing population and provide the revenues to meet its promises.
Institute executive chairman Azeem Ibrahim said: "Just how many immigrants we will need to come to Scotland every year from now till 2035 is a complicated issue with many variables.
"But assuming all other things remain equal, we will need not a 2.5% but an 8% increase in the population of Scotland in the next two decades - or in other words, we need 20,000 more immigrants per annum than we are currently expecting," Mr Ibrahim said.
The Scotland Institute added that an influx of immigrants into Scotland could lead to England introducing additional border controls.
Mr Ibrahim warned: "The rest of the UK may not be able or willing to support a Common Travel Area policy with Scotland as currently exists between the UK and Ireland. And if that happens, we will see border guards along Hadrian's Wall before too long."
One story dominates Thursday's front pages with some newspapers printing emotive pleas to voters to keep the United Kingdom as one.
In Edinburgh tonight you can hear it and feel it. The buzz, the banter of the day and the thrilling sense something big is coming tomorrow.
There is nervousness, anxiety and on the streets of Edinburgh fuelled by an atmosphere unlike anything seen before in recent UK politics.