Japan's ambassador to Beijing was summoned by China to an emergency meeting after former Japanese Prime Minister Shinzo Abe said Japan would not stand by if China took measures to reclaim Taiwan.
The ambassador was summoned by China's Foreign Ministry on Wednesday, and Chinese Assistant Foreign Minister Hua Chunying protested that Abe's warning had "openly challenged China's sovereignty and gave brazen support to Taiwan independence forces."
A Foreign Ministry statement said China was "resolutely opposed" to Abe's comments and had made "stern representations" to Japan.
The controversy was sparked after Abe issued a warning to Chinese President Xi Jinping at a forum for the Institute for National Policy Research - a Taiwanese think tank - on Wednesday.
In his speech, Abe said "a Taiwan emergency is a Japanese emergency, and therefore an emergency for the Japan-US alliance," before warning that China would be committing "economic suicide" if it attempts to take military action against the island.
Abe served as prime minister between December 2012 and September 2020, before being succeeded by Yoshihide Suga and most recently Fumio Kishida.
Beijing sees Taiwan as an inalienable part of its territory under its One-China policy, while the island's pro-independence leaders have enjoyed backing from Washington, with Taipei hosting high-profile US delegations on a regular basis. Tokyo has consistently voiced support for the status-quo in Taiwan Strait, sounding the alarm over "a sense of crisis" in the region. In its latest white paper, the Japanese Ministry of Defense called maintaining stability and security in the region one of Japan's top priorities.
Tokyo has also been embroiled in a long-running territorial dispute with Japan over the Senkaku Islands.