After a slow start, mobile WiMax could attract more than 2.5 million users in South Korea by 2011, but analysts warn that the lack of a voice call service is holding the technology back.

"The availability of VoIP will have a significant influence on subscriber addition," said Ku Kang of the ROA Group in a new report.

"But this will take time as it is unlikely that VoIP will be supported in the near future due to legal and other issues in the South Korean wireless market."

Korea was the world's first and largest test market for mobile WiMax with its WiBro service which was first made available in June 2006.

The service is expected to have 250,000 users by the end of this year, up from about 100,000 in 2007.

"Currently, the biggest disadvantage with WiBro is that it fails to provide a killer application," said Kang.

"Voice support is a must if WiBro is to become a 4G mobile technology, and it is a necessary element for competing with HSDPA and its next generation version, LTE, in the 3G/4G mobile market."

ROA predicts that, if the market can reach 2.5 million users, it will have a value of around $62m a year.

Korean government researchers are working on upgraded versions of WiBro which offer a maximum possible bandwidth of 3.6Gbps to users moving at low speeds, and up to 100Mbps to users moving at speeds up to 120kph.