The usernames of 22 million people may have been stolen from Yahoo Japan.

"We don't know if the file (of 22 million usernames) was leaked or not," admitted the company in an AFP statement. "But we can't deny the possibility, given the volume of traffic between our server and external" terminals.

 With around 200 million members, the loss of data represents more than 10 percent of all Yahoo Japan's users. Yahoo is Japan's most popular search engine, with a 50 percent market share compared with Google's 40 percent, said Wired UK.

Despite usernames alone not being enough to compromise an account, Yahoo Japan has advised users to change their passwords. The company says it has tightened its security to prevent a similar attack in future.