Roughly 14 percent of bulk e-mail ads received by mobile phones are illegal because they do not indicate they are unsolicited ads, according to a survey by NTT DoCoMo Inc.

Under legislation that took effect July 1, senders of the ads, known in the industry as spam, must attach messages explaining they are unsolicited and tell recipients how to reject them. The regulations also prohibit senders from resending the ads after individuals have rejected them.

According to the survey, conducted in August on 270 mobile phones, the ratio of illegal e-mail ads to legal ones remained unchanged in September, DoCoMo officials said, indicating the new legislation is too weak to fight spam.

The telecommunications ministry is now considering tougher rules to fight spam.

That ministry and the Ministry of Economy, Trade and Industry have received about 30,000 complaints about unsolicited e-mail and issued more than 1,000 warnings to the senders, officials from the ministries said.

"We need to take substantial action against malicious businesses that repeatedly send illegal e-mail ads, even after they receive warnings," a telecom ministry official said.

Under the new legislation, individual violators may face prison terms of up to two years or a fine of up to 3 million yen.

Corporate violators may face fines of up to 300 million yen.

The legislation was prompted by growing complaints from mobile phone users about unsolicited junk mail sent from mass-mailing firms.

The legislation comprises a new law regulating advertisement e-mail and an amendment to a 1976 law governing mail-order business.