Tax officials investigating companies and individuals linked to the Ansbacher deposits are encountering significant difficulties because so many Revenue files are missing or have been destroyed, according to sources.

Huge numbers of files were destroyed following the transition to self assessment in 1988 and again in a major culling in 1996.

Sources close to the Revenue said that in order to show that a particular company or individual made an incomplete return on a certain year, the file for that year is needed both so as to show the amount paid and the backing information which led to that figure.

However with many of the Ansbacher cases now being investigated by the special team appointed to do so, this crucial information is not available. The Ansbacher deposits have been in existence since the 1970s. "It's just impossible to find the files," said one source.

A spokesman for the Revenue, however, said that the archive was "quite good". He also pointed out that there were other sources of information, such as the records of financial institutions, and the report of the authorised officer who investigated the Ansbacher deposits, Mr Gerry Ryan.

Companies are required to keep details of their tax returns for six years after the returns are made. Evidence heard by the Moriarty tribunal indicates that the records of the main banks become very incomplete after varying periods of about 10 years.

While Guinness & Mahon bank has extensive, microfiche records, this is not the case with the secret "memorandum accounts" kept by the late Mr Des Traynor and which monitored who owned what in the Ansbacher deposits.

In April the chairman of the Revenue, Mr Dermot Quigley, told the Dail Committee on Public Accounts that the Revenue has a "major problem" in dealing with millions of taxpayers in terms of record storage and retrieval.

"There is a document retention policy but it is subject to management decisions about periodic culling of documents for reasons of efficiency and because there are storage problems."

Mr Quigley said there was a "culling" in 1996 involving a significant number of files. However he said he felt "reasonably confident" Revenue can address the cases which are arising out of the tribunals and the various inquiries.

There were the "lists" which were received from the authorised officer's inquiry, and also "voluminous information" from the financial institutions gathered by way of the Finance Act, 1999. "We also have other information from other third parties, and so on."

Mr Quigley said documentation would be requested from the taxpayer and, if the taxpayer did not co-operate, the Revenue would go to the courts.