Japan's Olympus admitted for the first time on Tuesday that controversial acquisitions had been used to cover up losses on securities investments dating back to the 1980s, succumbing to weeks of pressure that has battered the company's share price.

Olympus President Shuichi Takayama blamed Tsuyoshi Kikukawa, who quit as president and chairman on October 26, Vice-President Hisashi Mori and auditor Hideo Yamada for the transactions, adding he would consider criminal complaints against them if necessary. Mori would be dismissed, the company said.

I was absolutely unaware of the facts I am now explaining to you, Takayama told a press conference. The previous presentations were mistaken.

The revelations by the maker of endoscopes and cameras would appear to vindicate ex-CEO Michael Woodford, who has staged a public campaign since being sacked on October 14 to force the company to come clean on $1.3 billion in questionable transactions.

Olympus said it had found that funds related to its $2.2 billion purchase of British medical equipment maker Gyrus, which involved a massive advisory fee of $687 million, as well as three domestic firms were used to hide losses on the securities investments.

The disclosure leaves Olympus open to possible criminal charges for suspected accounting fraud and shareholder suits, lawyers and analysts said, putting the future of the 92-year-old company in doubt.

This is very serious. Olympus admitted it has made false entries to cover its losses for 20 years. All people involved in this over 20 years would be responsible, said Ryosuke Okazaki, chief investment officer at ITC Investment Partners. There is a serious danger that Olympus shares will be delisted. The future of the company is extremely dark.

The announcement sent Olympus shares tumbling 29 percent to a 16-year low on Tuesday. The company has lost 70 percent of its value, or $6 billion, since it fired Woodford.

It's big. Olympus was supposed to be a paragon of corporate society, said Darrel Whitten, managing director at Investor Networks Inc, an investor relations consultancy.

The company said in a statement the details of scheme came to light as part of its cooperation with a third-party panel set up to investigate the transactions. That panel was announced a week ago.

Takayama said he believed the loss-postponement scheme had started before the 1990s.

The company said it had funneled money related to the acquisitions through various funds and other measures to defer posting the losses, a practice seen in the days after Japan's bubble economy of soaring asset prices burst in 1990.

The scandal is the biggest in Japan since Livedoor entrepreneur Takafumi Horie's challenge to the business establishment ended with charges of securities fraud in 2006.

The members of the board appear to have breached their fiduciary duty owed to the company and to the shareholders, said Keiji Isaji, an attorney with K&L Gates law firm in Tokyo.

Lawyers said that if Olympus were found to have knowingly falsified its consolidated financial statements that were deemed material in nature, its representatives could face up to 10 years in prison or a fine of up to 10 million yen.

A spokesman for the Tokyo Stock Exchange said the bourse needed more information before deciding whether to put Olympus shares under supervision, a step toward possible delisting.

The TSE spokesman said it needed to examine the size of the deferred losses and whether they had an impact on shareholders' investment decisions before taking any further action.

Reuters reported last week that Olympus replaced its auditor in 2009 after a disagreement over how to account for the acquisitions.

In a confidential internal document obtained by Reuters, Kikukawa, the firm's then president, wrote to Olympus executives in the United States and Europe, revealing there had been a disagreement with auditor KPMG which he did not plan to disclose to the stock market.

In May 2009, Kikukawa announced the contract with KPMG had ended and that another global accounting firm, Ernst & Young, would take over. Kikukawa resigned on Oct 26. Executive Vice President Mori had been Kikukawa's right-hand man.

Olympus has come under increasing pressure to disclose more information to address shareholder concerns in an escalating scandal that has prompted law enforcement agencies in Japan and the United States to investigate.

The company suddenly fired Woodford on October 14, saying he failed to understand the company's management style or Japanese culture.

Woodford said he was forced out for questioning the $687 million paid for advice on the $2 billion Gyrus acquisition in 2008, the biggest fee in M&A history.

Woodford also questioned the acquisitions of three small Japanese firms whose value had been largely written off after the purchases.

Woodford told Reuters on Tuesday the Olympus board should resign. The position of the board and non execs is untenable now, Woodford said by phone from London.

He added that it was his desire to return to manage the endoscope maker should shareholders wish to reinstate him.

Prodded by institutional shareholders, Olympus has named six men, including a former Japanese supreme court justice, to investigate the past M&A deals.

($1 = 78.030 Japanese Yen)

(Additional reporting by Chikafumi Hodo, Tim Kelly; Antoni Slodkowski: Writing by Linda Sieg; Editing by Michael Watson, Edmund Klamann and Miyoung Kim and Dean Yates)