Yahoo has written to the United States director of national intelligence, James Clapper, requesting that the secret order that allegedly forced it to scan all user emails for terrorist communications is declassified.
The order first hit the news earlier this month, when reports said Yahoo had spied on user email on behalf of either the US National Security Agency (NSA) or the country's Federal Bureau of Investigation (FBI).
If proven, the allegations mean Yahoo will have scanned Australian and New Zealand customers' email as well those in other countries where the company is active.
Despite the damaging news reports, under US national security directives Yahoo cannot even divulge the existence of government classified orders.
Smarting from public criticism over the alleged spying and breach of users' privacy, Yahoo top legal officer Ron Bell took the unusual step of publishing a lettter [pdf] to Clapper, stating the internet portal was unable to respond to the accusations in detail because of the official gag on mentioning classified orders.
Bell is now asking Clapper to clarify if a classified order was issued to Yahoo.
He also wants Clapper's Office of the Director of National Intelligence (ODNI) to declassify the order wholly or partly if it exists, and to comment on the email spying allegations.
"We appreciate the need for confidentiality in certain aspects of investigations involving public safety or national security; however, transparency is criticial to ensure accountability," Bell wrote.
"In this context [it] must include disclosing how and under what set of circumstances the US government uses specific legal authorities, including the Foreign Intelligence Surveillance Act, to obtain private information about individuals' online activities and communications."
The alleged spying is believed to have led to the resignation of Yahoo's well-respected chief security officer Alex Stamos, who was not informed of the interception until after it had taken place.
Yahoo is currently for sale and in negotiations with US telco and internet provider Verizon over the deal.
US media reported that Verizon is now running cold on the purchase after the revelations of email spying, and is asking for a US$1 billion ($1.3 billion) discount on the price.