In a statement on Twitter, the Internet whistleblower said it was taking steps to sue, the BBC reported. The Guardian has responded by blaming WikiLeaks for a "security breach."
WikiLeaks has also been making more cables available, The New York Times said. Last week, it posted 134,000 pages of cables, a total higher than the disclosures of the past nine months.
U.S. prosecutors say WikiLeaks, founded by Australian Julian Assange, obtained the cables from Bradley Manning, a U.S. soldier now awaiting court martial. WikiLeaks worked with the Times, Guardian and other respected newspapers in releasing the first cables.
"A Guardian journalist has, in a previously undetected act of gross negligence or malice ... disclosed top secret decryption passwords to the entire, unredacted, WikiLeaks Cablegate archive," WikiLeaks said in its Twitter statement. "We have already spoken to the State Department and commenced pre-litigation action."
The Guardian said it had been assured by Assange personally a password revealed in a book published in February by two of its staffers was temporary and about to expire.
"No concerns were expressed when the book was published and if anyone at WikiLeaks had thought this compromised security, they have had seven months to remove the files," the newspaper said.
The release of classified cables infuriated U.S. officials and those in many other countries. It helped inspire at least one revolution when Tunisians read a U.S. diplomat's description of their president's lavish lifestyle.