Failure to hand control over National Insurance contributions to Holyrood as part of the wider package of devolution could cause conflict in future, a leading economist has told MSPs.
Professor Anton Muscatelli said he would prefer to see a "cleaner allocation" of all income tax and employment income powers.
The Smith Commission, set up to determine a new devolution settlement, recommended that National Insurance should remain reserved to Westminster, while Holyrood should be given control of income tax rates and bands.
But Professor Muscatelli told Holyrood's Devolution Committee that the set-up could cause clashes between the Scottish and UK governments because the two taxes are so closely linked.
"By not devolving National Insurance I think it creates potential for conflict, and it will impact on the Scottish tax base," he said.
"I think this is one of the issues why I would have preferred to see a cleaner allocation of all income tax and employment income powers to Scotland because it would have avoided these sorts of clashes.
"Here is an example where that interaction could create some difficulties in trying to extract who did what and what the impact is on the respective tax bases."
Charlotte Barbour, head of taxation at the Institute of Chartered Accountants of Scotland (ICAS), added: "They are all really finely inter-woven, the different taxes, and that includes National Insurance in my book."
Professor Muscatelli added: "One of the biggest issues around income inequality is around low paid in work, and a lot of that is to do with very high marginal tax rates.
"Supposing National Insurance had been devolved, it would have given Scotland an opportunity to look at that as a whole. Interestingly it might have triggered a similar review south of the border because of the fact that you might have had quite substantial divergence.
"However it is engineered, I would be in favour of such a change, because for the reasons that Charlotte has alluded, the two things (income tax and National Insurance) are becoming closer and closer together.
"The tax plus National Insurance structure in the UK is hugely complex."
A written submission to the committee from PricewaterhouseCoopers said that it did not consider the devolution of National Insurance to be appropriate due to its links to welfare spending, and for reasons of "legality and practicality".
National Insurance receipts contribute about 18% of tax revenues, the firm said.
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