London - British supermarket Tesco on Monday said it had launched an investigation after overstating its profit guidance for the six months to August by £250m ($408m).
"We have uncovered a serious issue and have responded accordingly," said chief executive Dave Lewis, who took up the position at the beginning of the month.
Audit firm Deloitte will head the investigation.
After the news broke early Monday shares in the company sank by 11% at one point, to an 11-year low.
On a conference call, Lewis said four members of staff had been suspended but refused to confirm reports that they included UK managing director Chris Bush.
The supermarket had already issued a profit warning on August 29, when it said profits in the six months to August 23 were expected to be around £1.1bn.
In a statement Monday it said this was overstated, "principally due due to the accelerated recognition of commercial income and delayed accrual of costs".