Although the crew were released about a week after the collision, the captain, who was arrested the day after the collision, was held for almost two more weeks, despite repeated protests from China.

The Foreign Ministry said in a public statement that the arrest 'seriously infringed upon China's territorial sovereignty'. Tensions escalated while China applied economic and diplomatic pressure on Japan leading up to the captain's release.

The Diaoyus, which is the latest matter classified by China as one of 'core national interest' - along with Tibet and Taiwan - were ceded along with Taiwan to Japan after the first Sino-Japanese war in 1895.

At the end of the second world war, the islands fell under the control of the United States. They were handed back to Japan under an agreement known as the 1971 Okinawa Reversion Treaty. This marked the start of strong protests from Beijing and Taipei, which both claim sovereignty over the islands.

Members of the Hong Kong-based Action Committee for Defending the Diaoyu Islands have organised campaigns supporting Chinese claims over the islands for more than a decade. During their most recent attempts to sail to the islands, the activists' boat, the Kai Fong II - also known as the Diaoyu II - was stopped by marine police two nights in a row, on September 22 and 23, just minutes before leaving Hong Kong waters and after being tailed for most of each day.

The Marine Department refused permission for the boat to leave Hong Kong waters. It said the boat could only be used for fishing and could not carry passengers.

But the activists said those on board were all crew members who intended to go fishing near the islands.

'We complied completely with the law,' claims David Ko Ke-hua, spokesman for the committee. All seven people on board, he says, were 'qualified to go without a doubt'. Three of them were the owner of boat, the captain and an engineer, and the other four were hired hands.

The veteran activist believes the recent action taken to stop members of the group from sailing to the islands has been motivated by political reasons.

Before the Marine Department took action, the trip was delayed by the Food and Environmental Hygiene Department, which conducted a rodent inspection on the vessel in mid-September.

The boat was also stopped from leaving Hong Kong waters by police and the Marine Department in May last year. The boat owner filed an application for judicial review.

However, the application was dismissed on the basis that the Marine Department director, in refusing the boat permission to leave Hong Kong waters with activists on board, was entitled to consider that the boat would not be used exclusively for fishing.

Members of the activist group have made several trips to the islands since 1996. Although some of the voyages were driven back by the Japanese Coast Guard, they had not been blocked by Hong Kong authorities until last year.

Activists and commentators have linked the government's apparent change of heart to political pressure exerted on Hong Kong due to the central government's unwillingness to hurt Sino-Japanese ties. China and Japan's economies are now the second and third largest in the world.

Political commentator Ma Ngok at Chinese University earlier told the South China Morning Post that 'on the one hand the central government wants to show an opposing voice to Japan, while on the other they fear some activities, such as sailing to the Diaoyu Islands, could bring conflict between the nations'.