Activists involved in the dispute over the Diaoyu Islands have been intercepted at sea four times this week by marine officers as authorities questioned the motive behind their movements.
The activists said they were planning to go to another disputed island chain, the Spratlys, to fish.
The foiled attempts led to threats of more action from both sides: the Marine Department, which would involve the police again if the activists' boat Kai Fung No2 violated a department order; and the activists, who would take the issue to court to "reclaim the right to fish".
Defending the ban yesterday, the government said: "The director of marine had grounds to believe Kai Fung No2 would not be used exclusively for fishing and fishing-related purposes."
The activists have sought to assert Chinese sovereignty over the group of eight uninhabited islands and rocks in the East China Sea, covering about 7 square kilometres in all, that Japan calls the Senkaku Islands.
Some of those on board this week were among 14 Chinese nationals arrested by the Japanese coastguard in August last year when part of the group landed on the main island and planted a Chinese flag.
It was the first landing by Hong Kong activists in 16 years.
Their arrests triggered a row between Beijing, Taipei and Tokyo, which all claim sovereignty over the island chain.
After being intercepted on Wednesday, the activists tried again a day later. They navigated towards the boundary of Hong Kong waters at about 11am and again at 5pm, when the department requested them to stop. Officers reminded the vessel owner of a directive issued by the marine chief. At about 9.15pm, Kai Fung No2 again approached the water boundary. Marine officers asked the vessel to stop and requested help from the police.
Marine police officers boarded the boat without the need to use any special equipment, and did not make any arrests.
The department said: "Since Kai Fung No2 has obtained the certificate of survey and an operating licence, it can be operated within Hong Kong waters. If the vessel violates the direction issued by the director of marine, the Marine Department will act in accordance with the law, and will request assistance from the police, if necessary."
Lo Hom-chau, of the Diaoyutai Islands Action Group, questioned the government's move and said they might turn to the courts to fight for their fishing rights.
Lo insisted they were trying to travel to the Spratly Islands in the South China Sea to fish.