Coalition ministers have warned that an independent Scotland would be forced to create its own financial services regulator – a move which could push up the costs of mortgages, insurance policies and pensions.
The Scottish Government's policy is to keep the current UK regulators.
Liberal Democrat Chief Secretary to the Treasury Danny Alexander told MPs that would be illegal under European Union law.
Instead, an independent Scotland would be forced to replicate a whole host of organisations, potentially costing millions.
The analysis will be contained in the UK Government's next policy paper on independence, focusing on financial services and expected to be launched in Edinburgh on Monday.
Mr Alexander told the Commons Scottish Affairs Committee: "Every independent sovereign state [in the EU] is required to have its own separate system of financial regulation and that is something that is just a fact."
Life insurance companies based in Scotland sell just 6% of their policies inside Scotland, with the rest to customers in other parts of the UK. Similarly, 78% of Scots life insurance policies are with companies based outside Scotland.
When it comes to mortgages, Scottish firms sell just 16% of their products in Scotland.
Scottish Secretary Michael Moore yesterday told the same committee he did not think a "shell" regulator –which shared many functions with the UK body – would be considered credible by the EU.
The SNP seized on his comments, under questioning from MPs, that he was "speculating".
SNP Treasury spokesman Stewart Hosie said: "Michael Moore has given the No campaign's game away. He has admitted the UK Government's position is nothing more than speculation and assertion. This admission fundamentally undermines the UK's case."
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