Legislator Albert Ho on Friday gave reporters a document that he said was a copy of the criminal verdict for the reporter, Shi Tao, from a court in the central province of Hunan.
"Yahoo Holdings (Hong Kong) Ltd provided materials that confirmed the user's information," the document said.
Ho, who is also a lawyer, said: "It's very clear from the judgment that Yahoo Hong Kong provided the details. They're a Hong Kong company. Why do they have to comply with Chinese requests (for information)? This is the biggest question."
The purported document appeared to contradict comments made earlier by Yahoo, which said evidence used to convict the journalist was provided by Yahoo's China unit.
Yahoo had said it was "distressed" when it learned of the facts surrounding the Shi Tao case, but stressed the company's China unit was merely complying with Chinese law.
Shi Tao, a former writer for the financial publication Contemporary Business News, was sentenced under state secrecy laws to 10 years in prison in April 2005 for allegedly providing state secrets to foreigners.
His conviction stemmed from an email he sent containing his notes on a government circular that spelled out restrictions on the media.
Ho and Shi Tao's friend, Zhang Yu, said that on Thursday they submitted a complaint against Yahoo Hong Kong to the territory's privacy watchdog, the Office of the Privacy Commissioner for Personal Data.
Journalists in China enjoy far less
freedom than those in Hong Kong
Ho passed out a copy of the complaint that said Yahoo Hong Kong "did not notify Shi Tao or ask for his permission" before it provided his personal information to Chinese authorities.
A spokeswoman at the Privacy Commissioner's office, who declined to be named citing operating policies, confirmed that the office received a complaint on Thursday related to Shi Tao.
The office was investigating the case, she said. She declined to provide further comment.
Ho said Yahoo Hong Kong did not reply to his requests for meetings. He said the company stressed it could not represent or take responsibility for Yahoo China's actions.
Hong Kong, a former British colony, has been governed under a "one country, two systems" formula since it returned to China in 1997.
The territory prides itself on having an independent rule of law and international business and privacy standards.