Hewlett-Packard Co on Tuesday took a massive $5 billion charge, claiming a raft of improprieties, misrepresentation and disclosure failures at software firm Autonomy, which it acquired last October for $11.1 billion.

HP said it discovered "serious accounting improprieties" and "a willful effort by Autonomy to mislead shareholders" after a whistleblower came forward.

The latest charge, which follows a nearly $11 billion charge last quarter for its EDS services division, is the latest blow to HP. The technology company has been roiled in the past few years by a revolving door of CEOs, overall management turnover and challenges in its core personal computer and printer businesses.

Former Autonomy Chief Executive Mike Lynch, who was pushed out in May, "flatly rejected" HP's allegations.

"The former management team of Autonomy was shocked to see this statement today, and flatly rejects these allegations, which are false," a Lynch spokeswoman said in a brief statement to Reuters.

HP took $8.8 billion in charges in the fourth quarter, with $5 billion tied to the problems at Autonomy.

HP said it has referred the matter to the U.S. Securities and Exchange Commission's enforcement division and the UK's Serious Fraud Office for civil and criminal investigation. It said it will take legal action to recoup "what we can for our shareholders."

HP informed both the SEC and the Serious Fraud Office over the past week. Both agencies declined to comment.

HP's stock slid to a 10-year low, losing 11.2 percent to $11.81 in afternoon trading. Shares are down nearly 50 percent year to date.

INFLATED SALES, REVENUE

HP alleged that Autonomy's former management inflated revenue and gross margins. It said Autonomy executives mischaracterized revenue from low-end hardware sales as software sales and booked some licensing deals with partners as revenue, even though no customer bought the product.

HP said it began an internal investigation, including a forensic review by PricewaterhouseCoopers of Autonomy's historical financial results, under HP General Counsel John Schultz after the whistleblower came forward.

Schultz said since the accounting troubles occurred prior to the acquisition, it took a long time before the company was in a position to make the news public.

"Not surprisingly, Autonomy did not have sitting on a shelf somewhere a set of well-maintained books that would walk you through what was actually happening from a financial perspective inside the company," he said. "Indeed critical documents were missing from the obvious places, and it required that we look in every nook and cranny."

HP CEO Meg Whitman said her predecessor, Leo Apotheker and the former chief strategy officer, Shane Robison, were the key people behind the Autonomy acquisition.

Apotheker was ousted as CEO in September 2011 after just 11 months on the job and Robison left soon after.

"Most of the board was here and voted for this deal, and we feel terribly about that," said Whitman on a call with analysts. "The board relied on audited financials, audited by Deloitte. Not Brand X accounting firm, but Deloitte," she said, adding that KPMG was hired to audit Deloitte.

"Neither of them saw what we now see after someone came forward to point us in the right direction," Whitman said.

Other advisers who worked on the deal included Qatalyst Partners, the investment bank run by technology investment banker Frank Quattrone; UBS; Goldman Sachs; Citigroup; JPMorgan Chase and Bank of America for Autonomy. Perella Weinberg Partners and Barclays Capital advised for HP.

Law firms for Autonomy were Slaughter & May and Morgan Lewis. The firms for HP included Gibson, Dunn & Crutcher; Freshfields Bruckhaus Deringer; Drinker Biddle & Reath; and Skadden, Arps, Slate, Meagher & Flom, which advised the board.

Lynch said he was "shocked to see" HP's allegations, adding that its due diligence prior to the acquisition was "intensive." He said HP's senior management was "closely involved with running Autonomy for the past year."

In response, Whitman said on CNBC the company stands by its findings.

In a statement, Apotheker said he was "stunned and disappointed" by the revelations and offered to make himself available to HP and the authorities to get to the bottom of the matter.

Robert Enderle, a tech analyst at the Enderle Group, said he has never seen such a potential misrepresentation of financials.

"You have to rely on what the firm gives you during due diligence and I've never seen a misstatement at this level," Enderle said.

If the charges are true, it could result in a massive punitive damages award for HP, Enderle said.

Other analysts hoped it was the end of the bad news for the company.

"This kind of feels like the last of the bad news," Forrester analyst Frank Gillett said.

FOURTH-QUARTER LOSS

The Autonomy allegations and announcement of the charge coincided with the reporting of a fourth-quarter loss for HP.

Net revenue fell 6.7 percent to $29.96 billion for the fourth quarter ended October 31 from $32.12 billion a year earlier. Analysts, on average, expected $30.43 billion, according to Thomson Reuters I/B/E/S.

Revenue from all of its main business units declined, with the personal computer division recording the steepest drop at 14 percent.

HP reported a quarterly net loss of $6.85 billion, or $3.49 a share, versus a profit of $239 million, or 12 cents, a year earlier.

The sprawling company, which employs more than 300,000 people globally, is undergoing a restructuring aimed at focusing on enterprise services in the mold of International Business Machines Corp.

"To put it bluntly ... this story has been an unmitigated train wreck, and it seems every time management speaks to the Street, there is new negative incremental information forthcoming," said ISI Group analyst Brian Marshall.