More than 20 per cent of NTT DoCoMo customers in Japan have signed up for flat rate mobile internet plans, the company has announced. 

The number of Japanese users opting to pay a fixed monthly rate for unlimited internet or mobile data use on their handsets has now passed 10 million, according to NTT DoCoMo.

"The plan has attracted great popularity by freeing users from concerns about data charges when enjoying data-intensive services," said the mobile carrier in a statement released this week.

NTT DoCoMo is using the data plans to tempt customers to try other profit generating services, such as a new flat rate music download service.

The cheapest flat rate data plan costs $34 a month, providing access to data services including email, messaging and DoCoMo's i-Mode.

The i-Mode network is an extensive, but private, mobile internet. It includes free information and news services as well as shopping, entertainment and other services billed directly to the user's mobile phone account.

Full mobile internet access costs $50 a month, but the service does not include PC internet access via the handset or smartphone, only through the device itself.

However, internet access through a mobile phone is considerably more popular in Japan than in most other markets.

The service was launched in March and NTT DoCoMo now has more than 51 million subscribers in Japan.

The i-Mode service, launched in the late 1990s, was the first successful mobile version of the internet, and succeeded at a time when Wap services were failing.

NTT DoCoMo was also the first carrier to set up a full W-CDMA 3G service in 2001.