The hacker collective Anonymous claims to have unearthed proposals by a consortium of US security firms to attack WikiLeaks, ahead of reportedly planned disclosures about the Bank of America.

Leaked emails apparently suggest that three private security firms – HBGary Federal, Palantir Technologies and Berico Technologies – pitched a plan to undermine the whistleblowers' site to a law firm which has represented the Bank of America. BoA, the largest US bank, is thought to be the next target of WikiLeaks releases.

Anonymous began releasing tens of thousands of emails sent by HBGary Federal late last week, after the loose-knit "hacktivist" group attacked the security firm's computer systems.

Aaron Barr, the company's chief executive, was targeted by Anonymous following a newspaper interview in which he claimed to be able to expose senior members of the shadowy internet collective.

One PowerPoint presentation found by Anoymous, and seen by the Guardian, apparently details a strategy to undermine WikiLeaks and its supporters by submitting falsified documents to the site and then exposing them as forgeries once they are published.

Another document sent by HBGary Federal to Hunton & Williams, the law firm which has represented the Bank of America, suggests ways to "mitigate [the] effect of adversarial groups" on the US Chamber of Commerce for a fee of $2m (£1.2m).

The leaked emails also show plans to undermine perceived supporters of WikiLeaks, including Salon.com reporter Glenn Greenwald and the Guardian's James Ball. "Without the support of people like Glenn, WikiLeaks would fold," the presentation said.

Greenwald said that his initial reaction to the leaked proposals was to "scoff at its absurdity", but "after learning a lot more over the last couple of days, I now take this more seriously – not in terms of my involvement but the broader implications this story highlights".

The Bank of America and the US Chamber of Commerce have said they had not known about the presentations, and that HB Gary Federal was never hired on their behalf. Berico Technologies and Palantir have also attempted to distance themselves from the leaked emails.

Guy Filippelli and Nick Hallam, the co-founders of Berico, confirmed they had been asked to develop a proposal to support a law firm which would "analyse internal information security and public relations challenges", but that the proposal was limited to "analysing publicly available information". It said that "pro-actively targeting" firms and individuals was "reprehensible".

Penny Leavy, the president of HBGary, an affiliate firm to HBGary Federal, on Monday said that the actions of Anonymous were "criminal".

"Anonymous should be regarded as the criminal group it is," Leavy told a security conference in San Francisco. "They have shown that they will go after security companies and anyone who works with the government."

Hunton & Williams had not responded to a request for comment at the time of publication. The US-based law firm could not be reached when contacted by the Guardian on Tuesday.

Anonymous also claims to have obtained access to Stuxnet, the sophisticated computer virus reportedly developed as a joint Israeli-US cyber attack against Iran's nuclear facilities, in HBGary Federal's computer system.