In one notable case, a wiretap under the Foreign Intelligence Surveillance Act (FISA) was disrupted due to an overdue bill.

"Late payments have resulted in telecommunications carriers actually disconnecting phone lines established to deliver surveillance results to the FBI, resulting in lost evidence, including an instance where delivery of intercept information required by a FISA order was halted due to untimely payment," according to the Justice Department's report.

The report was critical of the FBI's poor handling of money used in undercover investigations -- making the agency vulnerable to theft and fraud.

Although the FBI admitted 'a few instances' where late payment of phone bills had led to surveillance disruptions, they tried to play down their significance.

"These interruptions were temporary and in our assessment, none of those cases were significantly affected," said a spokesperson for the FBI.

The American Civil Liberties Union responded to the audit by highlighting the apparent hypocrisy of telecoms companies demanding immunity from prosecution over wiretaps -- in return for them acting as responsible corporate citizens.

"It seems the telecoms, who are claiming they were just being 'good patriots' when they allowed the Government to spy on us without warrants, are more than willing to pull the plug on national security investigations when the government falls behind on its bills," said Michael German, an attorney for the ACLU.

Investigations of the FBI's finances were initiated after an employee pleaded guilty to stealing more than US$25,000 (£12,750) in confidential case funds intended for wiretapping.

An FBI spokesperson admitted that there was 'widespread agreement' that its antiquated accounting system is inadequate -- but improvements are in the pipeline.

"The FBI will not tolerate financial mismanagement," they said.