Banks' use of the three-year loans also means current holders of bank debt would stand behind the ECB in the line to collect if a borrower collapsed, so "it has become even more expensive for banks to fund in the wholesale bond market, " said Ben Bennett, credit strategist at Legal and General Investment Management.

WSJ: European Banks��Large and Small��Rely on ECB

Even more vulnerable are stand-alone hedge funds that borrow heavily to dabble in debt.

ECONOMIST: There are losers, but some winners too

Two kinds of capital stand out: short-term debt, and debt denominated in foreign currency.

ECONOMIST: Shipbuilding

The current stand-off has its roots in a failed 2011 attempt to tackle the government debt limit and budget deficit.

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"The first thing he has to do is pay interest on his debt just to stand still, which means diverting profit which would otherwise have been reinvested in the club, " said Flitcroft.

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But it does little to address the first two issues: in fact, it makes another scary stand-off, over spending cuts and raising the amount of debt that the Federal government can legally issue, almost inevitable.

BBC: Cliffhanger: Still hanging