While some of its partners such as Anadarko and Transocean are covered to a certain extent by insurance, BP, like many of the major oil companies, has a negligible amount of its exposure insured, says Syme.

FORBES: BP Shares Rise On Takeover Talk

Government sources point out, though, that the amount of toxic debt being insured under APS is smaller than first estimated and RBS will take on a bigger share of potential losses.

BBC: Sell-off move leaves banks in flux

That level represents the amount of savings that are supposed to be insured if a bank collapses.

NPR: Cyprus Seeks To Alleviate Pain From Deposit Raid

Economists at Citigroup reckon that a fund ought to start with a baseline of 2% of insured deposits, and then top up that amount with an additional premium to reflect the risk that peripheral countries may leave the euro and that their currencies would then depreciate.

ECONOMIST: Preventing a big European bank run