The firm has been forced to delay its results even further

The firm, which has suffered a series of setbacks in recent months, said it would study how some revenues were accounted for over at least two years.

The firm has been in the spotlight since being blamed for delays to a �2bn upgrade of the NHS computer system.

Its chief executive quit last month and it is currently in talks with lenders about its finances.

Investigation

In a statement to the stock market, iSoft said it had received information suggesting possible irregularities in how it had treated revenue for accounting purposes in 2004-5 and at least one other year.

Concerns about this came to light during a recent audit following the adoption of a new accounting policy.

"The board has therefore commissioned an investigation into these possible irregularities,� the statement said.

The firm said it did not believe the investigation would significantly alter its revenues for last year, expected to be between �195m and �200m.

However, the firm has had to delay further the publication of its audited results for the past year, already put back several weeks.

Licence changes

ISoft's shares fell 40% last month after it revealed that sales and profits for the past year would be significantly lower than previously thought because of a change in accounting rules in relation to new licences.

Although the change has not affected its cash reserves, iSoft is currently in talks with its lenders about alterations to its banking arrangements.

Chief executive Tim Whiston resigned last month and was replaced by business turnaround specialist Bill Henry.

The firm also announced plans to cut up to 150 jobs.