The accounting irregularity led the company to issue today its third profit warning in two years. The announcements shocked investors, with shares falling 8.8 per cent to 209.40 pence in early trading today.
The investigation, prompted by information from a whistleblower, comes less than a month after the new chief executive, Dave Lewis, took charge.
The company has faced intense competition as retailers cut prices to attract customers hit by tough economic times.
Lewis took over from Philip Clarke after the company issued a profit warning at the end of August. At the time, Tesco said it expected to report "trading profit" of about 1.1 billion pounds for the six months ended August 23.
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The retailer now plans to release its earnings for the period on October 23, three weeks later than previously scheduled.
Tesco said the overstatement resulted from reporting commercial income too early and delays in booking some costs.
The company asked Deloitte to begin an independent review, along with the group's external legal advisers.
"This is about getting to the bottom of a full and frank inquiry of what happened," he said.
Shore Capital analyst Clive Black said the development is flabbergasting.
"Such an announcement is not the stuff of a well operated FTSE-100 organisation."
Neil Saunders, managing director of retail consultancy Conlumino, said that while mistakes happen, it gives the impression of a company which is not fully in control of internal procedures.
"More significantly, it means that performance, which is already extremely weak, is actually much weaker than anticipated," he said. "This is something that will alarm investors and means that Tesco has much further to travel to recovery than first thought.