Tesco's £250m accounting black hole first flagged during Phil Clarke's reign

Mr Clarke was replaced as Tesco chief executive on September 1 by Dave Lewis.

The retailer’s first half ended on August 23, nine days before Mr Clarke’s departure, but it is understood the whistleblower, who worked in Tesco’s accounting department, had already voiced concerns while Mr Clarke was still in post. According to The Sunday Times, the whistleblower's concerns "failed to get traction" when they were highlighted the first time round.

The supermarket giant admitted last week that it had “identified an overstatement of its profit for the half year”. Mr Lewis alerted the markets to the “serious issue” after the whistleblower contacted Tesco’s general counsel, who told Mr Lewis on Friday September 19. Tesco issued a statement three days later, when the stock market re-opened.

A Tesco spokesman said: “We have an investigation under way and we will await the findings before drawing any conclusions.”

After a torrid week for Tesco, its rival J Sainsbury will also get a taste of shareholder discontent this week when it is expected to unveil a plunge in like-for-like sales of up to 4pc over the summer.

Sainsbury’s has been under pressure from the German discount chains, Lidl and Aldi. The most recent market share data, from research firm Kantar, show its share of the UK grocery market slipped to 16.2pc from 16.6pc in the 12 weeks to September. Sainsbury’s has fought back with a fresh round of price cuts and matching its prices on branded products to Asda but the chain is coming under particular pressure from Aldi and Lidl in its key markets of the south and east of England.

Analysts are braced for a cut to the dividend while some have even raised the possibility that Sainsbury’s may have to raise extra cash through a rights issue.

Clive Black, analyst at Shore Capital said a “potentially significant cut to dividends could be on the way”.

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