Wal-Mart Stores, which last month gave federal prosecutors details of an internal probe into expense abuse by a former top executive, said yesterday that it was co-operating with a grand jury investigation.

The company, whose shares were down 2 per cent, declined to elaborate on the specifics of the grand jury investigation.

"We are aware that a grand jury is looking into the matter and are co-operating fully," the company said in a statement.

The company's internal probe looked into the personal expenses, payment of third-party invoices and use of company gift cards by prominent Irish-American and former vice-chairman Thomas Coughlin. Mr Coughlin, who retired as an executive in January, quit his position on the company's board of directors at Wal-Mart's request in March.

Wal-Mart said it passed on details of the internal investigation to the US Attorney for the Western District of Arkansas, including information from Mr Coughlin about his knowledge of certain transactions.