The UK's accounting watchdog has launched an investigation into a £263m overstatement of profit expectations at Tesco.
A probe by the Financial Reporting Council (FRC) will focus on financial figures reported and prepared by Britain's biggest supermarket and its auditor, PwC.
The scandal is already being investigated by the Serious Fraud Office and has seen a number of Tesco executives suspended - some of whom have now left. Chairman Richard Broadbent has also announced he is to step down.
Tesco first announced in September that it had discovered an accounting problem relating to expected results for the first half of the current financial year.
A subsequent probe by Deloitte into the affair, which involved rebates from suppliers being moved to different periods on the company's balance sheet, found it had been going on at least as far back as the 2012/13 period.
The FRC has already said it was "giving careful consideration" as to whether it should take regulatory action.
Today the body said it had launched a probe "in relation to the preparation, approval and audit" of Tesco's financial statements in the financial years ending in 2012, 2013 and 2014.
It will also cover "conduct in relation to the matters reported in the company's interim results" for the first half of the current financial year - when the Deloitte probe was published.
The investigation will cover members of professional accountancy bodies, from either Tesco or its auditors, who were involved with the accounts, as well as the member firm, PwC, which was and remains Tesco's auditor.
"We take our responsibilities very seriously and remain committed to delivering work to the highest professional standards. We will cooperate fully with the FRC in its inquiries," PwC said in a statement.
The announcement opens up Tesco's books to scrutiny going back a year further than the previous investigation by Deloitte.
Results of that probe published alongside half-year figures this year showed a £118m hit for the latest period, plus £70m for 2013/14 and £75m for 2012/13. But the FRC investigation goes back to the year ended 2012.
The FRC has powers to issue unlimited fines to individuals or firms or to suspend them from professional accounting bodies.
"We note the Financial Reporting Council is launching an investigation into individuals and a member firm in relation to the preparation, approval and audit of our accounts for the last three years," a Tesco spokesman said.
"We will provide support to the FRC's investigation," it added.
It is the latest challenge facing the supermarket coming a couple of weeks after it issued a £500m profits warning - its fourth this year.
Tesco said earnings would be hit by investments in price and availability of its products as well as a shake-up of the way it handles its supply chain in the wake of the accounting scandal.
Chief executive Dave Lewis, who was appointed in September, has temporarily taken charge of the supermarket's UK operations during its key Christmas trading period. Sales have been squeezed amid a price war spurred by the threat of discounters Aldi and Lidl.
Eight executives including UK managing director Chris Bush were suspended in the wake of the accounting scandal.
Mr Bush and three others have now left, one has returned to his job and three remain suspended. Mr Lewis has said the suspensions do not indicate guilt.
Tesco shares have plunged more than 40% over the last year and after a mini-rally when they rose 5% on Friday were in the red again today.