A US federal court document asking a judge to seal documents in a criminal case unrelated to Mr Assange inadvertently revealed the charges against the Australian.
The document, which prosecutors say was filed by mistake, carries markings indicating it was originally filed in US District Court in Alexandria, Virginia in August.
A source familiar with the matter said the document was initially sealed, but was unsealed this week for reasons that are unclear at the moment.
Wikileaks tweeted the revelation was an “apparent cut-and-paste error”.
US officials had no comment on the disclosure in the document about a sealed indictment of Mr Assange and it is unclear what charges he faces.
But Joshua Stueve, a spokesman for the prosecutors’ office that filed the document that was unsealed, admitted the court filing was an “error”.
“That was not the intended name for this filing,” he said.
According to the document, prosecutors sought to keep the charges confidential until after Mr Assange’s arrest, saying the move was essential to ensure he did not evade or avoid arrest and extradition in the case.
“[Anything] short of sealing will not adequately protect the needs of law enforcement at this time because, due to the sophistication of the defendant, and the publicity surrounding the case, no other procedure is likely to keep confidential the fact that Assange has been charged,” the document read.
“The complaint, supporting affidavit and arrest warrant, as well as this motion and the proposed order, would need to remain sealed until Assange is arrested in connection with the charges in the criminal complaint and can therefore no longer evade or avoid arrest and extradition in this matter.”
US officials have previously acknowledged that federal prosecutors based in Alexandria have been conducting a lengthy criminal investigation into WikiLeaks and its founder.
Representatives of President Donald Trump’s administration, including Secretary of State Mike Pompeo, have publicly called for Mr Assange to be aggressively prosecuted.
Mr Assange and his supporters have periodically said US authorities had filed secret criminal charges against him, an assertion against which some US officials pushed back until recently.
Facing extradition from Britain to Sweden to be questioned in a sexual molestation case, Assange six years ago took refuge in Ecuador’s London embassy, where initially he was treated as a welcome guest.
But after a change in the government of the South American nation, Ecuadorean authorities last March began to crack down on his access to outsiders and for a time cut off his internet access.
Mr Assange and his lawyers are yet to comment publicly.