Outgoing CEO Sergio Rial, who replaced Miguel Gutierrez less than two weeks ago, said in a conference call with investors he had "clearly found some very important distortions" and signals of a lack of transparency from the previous management.
In a pre-market auction that lasted most of the session, the stock dropped as much as 90%, before opening down 76.25% at 2.85 reais.
Chief Financial Officer Andre Covre, who had just joined Americanas as well, also left the firm long controlled by three Brazilian billionaires who founded 3G Capital.
Rial attributed the inconsistencies to differences in accounting for the financial cost of bank loans and debt with suppliers.
The outgoing CEO said Americanas would likely need a capital increase, though noted he didn't expect a short-term impact from the inconsistencies on its cash position.
Several analysts put their recommendations on the firm under review, while Brazil's securities regulator CVM said it had started two probes into the retailer to investigate the issues.
Separately, Americanas said an independent committee was being formed to investigate the matter that would include former CVM director Otavio Yazbek.
Earlier in the day, XP Investimentos, Bradesco BBI, Banco Safra and Itau BBA all placed Americanas under review following the news, while analysts at Morgan Stanley moved it to "Not-Rated" from "Overweight," saying an expected operating turnaround had become clouded.
Americanas in a securities filing late on Wednesday said it estimated the cash impact of the inconsistencies was not material, although internal inquiries and work by independent auditors would be needed to determine the impact of them on its financial statements.
"Even after the meeting with the company to clarify points mentioned in the filing we still do not have enough visibility to detail the financial impact the revisions should have on the company's figures," analysts at XP Investimentos said.
Analysts at Santander and JPMorgan, who rated Americanas "Neutral" and "Underweight" respectively, also forecast a major negative reaction to the news.
"In sum, we see the company's cash flow situation, which was the reason why we recently downgraded the stock to UW, as much more delicate than anticipated," JPMorgan said.
PwC, Americanas' auditor, declined to comment on the accounting inconsistencies referred to by Rial.
($1 = 5.1600 reais)
(Reporting by Paula Arend Laier, Gabriel Araujo, Tatiana Bautzer, Alberto Alerigi Jr. and Aluisio Alves; Editing by Conor Humphries and Mark Potter)
By Paula Laier and Alberto Alerigi
