Japanese officials had grown increasingly concerned as Beijing escalated the dispute, sharply curbing government exchanges and threatening further “countermeasures."
China’s announcement on Thursday that it was investigating four Japanese citizens for “illegally” filming a military site was also interpreted as an effort to increase the pressure for the release of Mr Zhan.
In announcing on Friday that Mr Zhan would be returned to China, Japanese prosecutors in the southern prefecture of Okinawa made clear that the diplomatic dispute had been a factor in their decision.
“Considering the implications for citizens of our nation and future Japan-China relations ... it was not appropriate to continue to hold the captain in custody for further investigation,” a spokesman said.
The release of Mr Zhan should mark the end of what had become the biggest diplomatic setback suffered by the government of Naoto Kan, Japan’s prime minister, since he took office three months ago.
However, it is also sure to expose Mr Kan and the ruling Democratic party to accusations that it bowed to pressure from Japan’s giant communist neighbour.
Seiji Maehara, Japan’s new foreign minister who is relatively hawkish on China, had repeatedly stressed that the fishing boat was operating in Japanese territory and that legal action against the captain should be handled according to domestic law.
In his previous role as transport and infrastructure minister, where he was responsible for the coast guard, Mr Maehara went to see the damage to two of the service’s vessels in collisions that Japanese officials say were caused by the fishing boat.
Mr Zhan could still be formally charged over the collisions. However, prosecutors said that other factors in the decision to release the captain included a judgement that his actions were not premeditated, and recognition that nobody had been injured in the incident and that the coast guard patrol boats had not been rendered inoperable.
Japanese officials had been dismayed by the escalation in China’s reaction, which included halting top level government contacts and the cancellation of an invitation by Mr Wen to 1,000 young Japanese to visit the Shanghai Expo this week.
Concerns grew further when Xinhua, the official Chinese news agency, on Thursday said state security in Hebei province had taken “measures” against four Japanese citizens “according to law after receiving a report about their illegal activities.”
Xinhua said the four had trespassed and filmed military sites “in recent days.” China has many military zones scattered around the country, some of them poorly marked, making involuntary trespass comparatively easy.
Fujita, the Japanese construction company, on Friday said it had been informed that four of its employees had been detained by Chinese authorities. The company said it had not been in touch with the employees since Wednesday.
Kyodo news agency said the four men had reportedly been visiting China’s northern Hebei Province to “prepare for bidding on a project to dispose of chemical weapons abandoned in China by the Japanese military at the end of World War II.”