NTT DoCoMo Inc. officially announced Wednesday it will introduce a flat-rate fee for e-mail and Internet services on mobile phones June 1 that will undercut those of rival KDDI Corp.

The country's largest mobile phone operator said it will let users of FOMA third-generation services use its i-mode service at a fixed rate of 3,900 yen per month.

The company's move came after KDDI in November launched a flat-rate program for au mobile phone e-mail and Internet, charging 4,200 yen per month.

NTT DoCoMo's new service, however, will be limited to those who subscribe to monthly basic fee plans of 6,700 yen or more and pay the flat-rate fee of 3,900 yen.