Scots would remain liable for the
full amount if the Westminster Government refused to write off the debt, they warned.
The sources also said that would leave the money owed hanging over the new country, adversely affecting its credit rating.
Conservative insiders also made the most explicit threat yet that reneging on the debt could see an independent Scotland lose out on billions of pounds worth of joint assets, such as embassies or fighter jets.
Suggesting the two could not be viewed separately, a source said: "If you won't pay the mortgage, why should you get to keep the house?"
The warnings raise the stakes in Edinburgh and London's war over the future of both the pound and the national debt.
The Scottish Government said the position on debt was "crystal clear" and that all responsibility for it lay with the UK Government. The SNP insist it is the pro-Union parties who are refusing to share a joint asset, sterling.
The First Minister has threatened to reject any share of national debt if London blocks his plans to share the pound in a formal currency union, saying Scotland cannot accept liabilities without the associated asset.
The Treasury has pledged to honour all UK Government debt, a move widely seen as an attempt to head off market turmoil in the event of a possible Yes vote. But a source said that while the UK would publicly stand behind the
debt, that did not preclude "a separate debt - that Scotland owed the UK, whether Alex Salmond agreed to it or not."
He added: "I don't think Alex Salmond can give up the debt".
David Owen, an economist at Jeffries International, a leading global investment bank, said: "The UK Treasury could decide to keep the debt on the books as something that the Scotland would owe, at some point".
He added that refusing to pay the debt would be seen by the markets as "technician default" and mean Scotland would have to pay more to borrow money internationally.
In January the Treasury said an independent Scottish state would be responsible for a "fair and proportionate share of the UK's current liabilities". The exact share and the terms of repayment would be subject to negotiation, it added.
The National Asset Register published in 2007 estimated the UK's fixed assets to be worth around £337 billion, suggesting Scotland's share could be more than £30bn.
The First Minister has accused the Conservatives, Liberal Democrats and Labour of "bluffing" over their threat to reject a sterling zone with an independent Scotland.
Professor Joseph Stiglitz, the Nobel prize winning economist and Columbia University academic, a member of Alex Salmond's Fiscal Commission expert panel, has said the move is "obviously" a bargaining position.
But Crawford Beveridge, the chairman of the same panel and the former head of Scottish Enterprise, said earlier this week that the parties could follow through on that threat.
A Scottish Government source said: "The position on debt is crystal clear - the UK Treasury issued a statement in January this year making clear it will accept legal liability for all UK debt under all constitutional circumstances. Scotland should agree to finance a fair share of that debt - on the basis that we are also entitled to a fair share of assets."
The Treasury said the UK Government would not pre-negotiate ahead of the result of the independence poll.