Creditors of a $30 billion (£15.2 billion) finance company managed by Gordian Knot, the City investment firm, have called in Deloitte, the accountants, to advise them on recovering their cash amid fears that Sigma Financial may not be able to pay its debts.

Deloitte, which has been at the forefront of overhauling structured investment vehicles (SIVs) that went bust in the credit crunch, confirmed last night that it had been approached by an informal committee of Sigma creditors.

Sigma has not run out of cash and, unlike most SIVs, does not have triggers that force it to wind up if the market value of its assets falls below a set point. However, it has $8.6 billion of debt maturing by September 30 and investors are thought to be worried about Sigma's ability to meet its obligations. On Friday analysts at Citigroup said that Sigma was “walking a tightrope” with its funding arrangements.

Gordian Knot was set up in 1993 by Nicholas Sossidis and Stephen Patridge-Hicks, formerly of Citigroup. It refused to comment yesterday.

SIVs sold short-term commercial paper and invested the cash in longer-term assets, such as mortgage-backed securities. At its height, it was a $400 billion market used by banks and hedge funds to put cheap debt to work on higher-yielding investments.

The global liquidity squeeze meant that investors became reluctant to buy the vehicles' debt, while the SIVs' investments plunged in value. As a result, the SIV market has shrunk in value to $110 billion, at most.

Some SIVs were moved back on to balance sheets by the banks and investment companies that set them up. Other SIVs were not restructured quickly enough to avoid hitting the market value triggers that forced them to liquidate.

Deloitte worked with Goldman Sachs, the investment bank, for ten months to create a model for restructuring SIVs and has completed two: the $7 billion Cheyne Finance and $2.4 billion Rhinebridge. The accountancy firm is still working on restructuring Whistlejacket, a $7 billion SIV formerly managed by Standard Chartered, and the $1.9 billion Golden Key, which are not expected to be completed until autumn.

Unlike other SIVs, Sigma did not invest in US sub-prime mortgages. It also has funding sources other than its asset-backed securities to repay investors - it raises money by using assets such as bank debt as collateral to borrow from other banks and investors. This is referred to as a repo.

However, this leaves Sigma's senior creditors with their debts secured on less popular collateralised debt obligations (CDOs) and commercial mortgage-backed securities. In a note on Friday, Citigroup analysts said: “Should any of the repo counterparties withdraw its funding, there does not seem to be much room for manoeuvre.”

Last month Moody's, the ratings agency, downgraded Sigma's senior debt from A2 to A3 and put the company on review for another downgrade. Standard & Poor's downgraded it in April and put it on ratings watch negative.

Fitch withdrew its rating in January after a disagreement with Gordian Knot over the agency's decision to put the SIV on a negative ratings watch.

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