China has warned Japan that its decision to grant a sight-seeing visa to former Taiwan President Lee Teng-hui will harm bilateral relations.
China views Mr Lee as an agitator for Taiwan independence
Mr Lee edged Taiwan towards formal independence during his 12 years as president, infuriating Beijing, which sees Taiwan as a breakaway province.
Japan insisted its granting him a visa was not politically motivated.
But it comes during a chill in relations between the two, who clash over history and resources.
"We think Japan's agreeing to let him [Mr Lee] visit is itself a challenge to China's unification efforts and is a type of support for and indulgence of Taiwan's independence forces," Chinese Foreign Ministry spokesman Liu Jianchao told a news conference.
"We are resolutely opposed and requested the Japanese side to immediately cancel this decision or else it will certainly create new influences on China-Japan relations," he said.
Japanese Prime Minister Junichiro Koizumi denied that the decision was designed to irritate Beijing.
"I regard Japan-China relations as significantly important," he told reporters.
His Chief Cabinet Secretary, Hiroyuki Hosoda, said Japan had not changed its position on the Taiwan issue, and did not support Taiwan independence.
The last time Mr Lee visited Japan was in 2001 for medical treatment, on what Tokyo said were humanitarian grounds. That also sparked Chinese anger.
Relations between China and Japan have been soured in recent weeks by an alleged incursion by a Chinese submarine into Japanese waters, and a continuing row over repeated visits by Japanese Prime Minister Junichiro Koizumi to the country's Yasukuni Shrine.
The shrine is dedicated to the souls of the country's war dead, including convicted war criminals, and is viewed by other Asian nations as a symbol of Japanese wartime aggression.