IT would be “wrong” to use population size to work out an independent Scotland’s share of the UK national debt, according to a leading economist.

The UK’s borrowings are estimated at £1.3 trillion. And the pain of paying back a multi-billion sum has been used as an argument against independence. In 2014, Westminster said an independent Scottish state “would become responsible for a fair and proportionate share of the UK’s current liabilities” and “would need to raise funds in order to reimburse the continuing UK for this share”.

Set by population size, that could make Scotland’s share around £106 billion. However, writing in The National today, Richard Murphy, professor of practice in international political economy at City, University of London and director of Tax Research UK, says this “simple calculation” would be “wrong”.

He argues that, in the event of a Yes vote, Scotland’s population “could and should be weighted” by age, income, wealth level, GDP or other factors.

And he says it’s questionable whether Scotland would really owe anything at all – because of the massive boost North Sea oil has given the UK economy.

READ MORE: What scale of debt could Scotland inherit in event of independence?

Murphy states: “An older population should not be required to assume so much debt. It could be weighted by wealth as well – why should a poorer population be expected to assume as large a part of the debt as a richer one? Apportionment on the basis of average income would obviously be another factor since debt outstanding does represent tax uncollected despite government spending having arisen, and we supposedly have a progressive tax system, meaning that the richer country should be liable for more of the debt as a result.

“GDP could be another basis, excepting the fact that there is no reliable estimate for Scottish GDP. I am not doing those calculations here. I am just suggesting they could be done. I do not think there is the remotest chance that a simple apportionment could get accepted as fair.”

The comments come in the first of a two-part series carried by this newspaper. The second, which will run tomorrow, will focus on “whether Scotland has actually generated this debt”, as well as the potential repayment window and the issue of how interest could be calculated.

Setting this up, Murphy writes: “The question comes down to oil.

“Given the phenomenal contribution North Sea oil made to the UK as a whole, which it can reasonable be argued was squandered outside Scotland, does Scotland have any liability for the debt, come what may?”

Callum Baird, editor of The National, said: “Richard Murphy’s analysis is a must-read for anyone interested in Scotland’s future.”