The search portal provided information to Chinese authorities that helped identify and prosecute several journalists and pro-democracy writers.
Shi Tao was sentenced to 10 years in prison for forwarding a government email to the foreign press. The authorities had tried to warn journalists of the 'danger' posed by the return of dissidents on the 15th anniversary of the Tiananmen Square massacre.
Jiang Lijun was sentenced to four years in November 2003 after Yahoo! provided information that helped identify him. He was sentenced for writing articles that called the Chinese government "autocratic" and said that he favoured a western-style democracy.
Li Zhi was sentenced to eight years for discussing pro-democracy issues in a web forum and for emailing pro-democracy campaigners.
"The Chinese government has an atrocious record of censorship and free expression and it is essential that the rest of the world publicly objects to and campaigns against this repressive regime," said Jeremy Dear, general secretary of the NUJ.
"The NUJ regards Yahoo!'s actions as a completely unacceptable endorsement of the Chinese authorities. As a result, the NUJ will be cancelling all Yahoo!-operated services and advising all members to boycott Yahoo! until the company changes its irresponsible and unethical policy."
Members are advised to contact the new media council for advice on alternative email, group forum and portal services.