Three men have been imprisoned for 21 to 27 months for making and selling pirated television set-top boxes, enabling users to watch World Cup broadcasts and other programs illegally.

The prison terms were the harshest sentences handed down against online copyright infringement in Hong Kong.

But it only marks the start of a long battle as there remains many similar set-top boxes sold in Hong Kong and on online shopping platforms, the Coalition Against Piracy said.

The judgment will help "clarify" that many of these devices are illegal, it added.

Customs officers busted the set-top boxes syndicate three years ago when 2014 World Cup matches were being staged.

Two technicians subscribed and configured 11 paid channels from NowTV and retransmitted them through a mainland server.

Maige TV Box, the illicit streaming device, then offered direct links to the stolen channels.

Chung Yu-cheung, 39, was given a 27-month jail term, while fellow technician Ho Kwok-cheung, 53, and seller Lam Siu-wai, 53, were both jailed for 21 months.

The three pleaded guilty in November to six charges -including providing a circumvention device or service and conspiracy to defraud - marking the first convictions involving illicit set-top boxes.

District Judge Gary Lam Kar-yan said the three's transmissions had adversely affected NowTV.

"The company is facing a HK$4 million potential loss of revenue," he said.

NowTV's price list shows that paid channels and packages cost from HK$10 to HK$428 per month.

Maige TV Box was sold for HK$2,200 but users get to watch films, TV shows and live sports for free.

Yip Wai-sim, head of the Customs' Intellectual Property Investigation Bureau, said the ruling will serve as "a strong deterrent." Now TV said it is a victory for intellectual property rights and the creative industry.

Desmond Chan Shu-hung, TVB deputy general manager for legal and international operations, said: "Hong Kong's TV industry has suffered a huge revenue loss due to illicit streaming for many years."

Neil Gane, general manager of Coalition Against Piracy, said illicit streaming devices with preloaded applications are readily available in many electronic outlets in Hong Kong.

Information technology lawmaker Charles Mok cited the verdict's significance, but said it is only a small victory in the battle against illicit streaming.