Hacking network Anonymous has released a recording of a conference call between the FBI and UK police in which they discuss efforts against hackers.
The call, said to have taken place last month, covers the tracking of Anonymous and similar groups, dates of planned arrests and details of evidence held.
Anonymous also published an email, apparently from the FBI, showing the email addresses of call participants.
The FBI and Scotland Yard confirmed the leak and said they were investigating.
It also emerged on Friday that hackers linked to Anonymous had accessed the websites of several US law enforcement agencies and that of the Greek justice ministry.
The FBI said in a statement: "The information was intended for law enforcement officers only and was illegally obtained. A criminal investigation is under way to identify and hold accountable those responsible."
London's Metropolitan Police's central e-crime unit said the matter was being investigated but that no operational risks had been identified.
It was unclear how Anonymous had managed to obtain the recording but a lawyer for one of the suspects discussed told the BBC it appeared to have been taken as an audiofile from an intercepted email, rather than having been eavesdropped on.
A comment on one of the Twitter accounts linked to Anonymous, AnonymousIRC, said: "The FBI might be curious how we're able to continuously read their internal comms for some time now."
BBC security correspondent Frank Gardner says the recording, which was published on YouTube, will be highly embarrassing for the cyber crime detectives.
At one point an FBI agent thanks the Metropolitan police for their help with investigations, to which the British detective replies: "We're here to help. We've cocked things up in the past, we know that."
According to the alleged leaked email, the 17-minute phone call took place on 17 January.
The email was sent to law enforcement officials in the US, UK, Sweden, Ireland and other countries, inviting them to "discuss the on-going investigations related to Anonymous, Lulzsec, Antisec, and other associated splinter groups".
The recording features the voices of about five men, apparently speaking from London, Los Angeles and Washington.
They begin with light-hearted conversation but move on to discussing the names of some of the people being tracked and their plans for legal action.
The online pseudonyms of suspects are included in the recording, but some of the real names appear to have been bleeped out.
Among those discussed are several British men accused of being behind cyber attacks in the US and UK, including Jake Davis and Ryan Cleary who were arrested last year.
The British detectives discuss delaying some arrests while US investigations are being carried out.
The police also refer to a 15-year-old who claims to have been behind an attack on online gaming site Steam last year, where the identities and credit card details of tens of thousands of users were accessed.
"He's a 15-year-old who's basically just doing this all for attention," says the British detective.
Anonymous is a loose collective of hackers, anarchists and pranksters which has targeted the websites of a range of governments, companies, law enforcement agencies and individuals in recent years.
Also on Friday, Anonymous claimed credit for an attack on a police website in the US city of Boston. A message posted on the site said it had been hacked several months ago in response to "police brutality" towards the Occcupy Wall Street movement.
This followed another attack on a police website in Salt Lake City on Tuesday, where officials say personal information on informants, crimes and residents were accessed.
Hackers operating under the Anonymous name also took over the website of Greece's justice ministry on Friday, prompting officials to take the site down.
The hackers said the action was a protest against Greece's signing of a global copyright treaty and the government's handling of the economic crisis.
The website was replaced with a video of a figure wearing the symbolic white mask of Anonymous supporters, saying: "Democracy was given birth in your country but you have killed it."