In an apparent bid to prevent customers from jumping ship to budget mobile phone services, SoftBank Corp. said Thursday it will cut monthly rates for its smartphone data plans.

A new plan dubbed the "Giga Monster" will offer 20 GB a month for ¥6,000 and 30 GB for ¥8,000 starting Sept. 13, down from ¥16,000 and ¥22,500, respectively.

Mobile phone users are consuming increasingly large amounts of data-rich content such as videos, so "SoftBank is looking to provide plans where customers won't have to worry about their data usage," it said in a statement.

SoftBank had been widely recommending the 5 GB plan, saying that 93 percent of its smartphone subscribers use 5 GB or less a month. But complaints that the plan falls short have been increasing.

SoftBank's move is also likely to increase competition with rivals Docomo and KDDI. SoftBank said the new plans were made possible by new technology that allows it maintain faster and steadier internet connections, which isn't something its competitors can emulate quickly.

The cheaper rates come at a time of relative calm in the mobile phone arena, a pause prompted by a request from the communications ministry to stop using major cash-back incentives to lure customers.

Still, SoftBank's move may also signal that it's feeling the pinch of customers switching to low-cost smartphone service operators known as mobile virtual network operators (MVNOs).

Subscribers to MVNOs surged to 5.39 million at the end of March, up 65.5 percent from a year ago, and their numbers are only expected grow, according to MM Research Institute.

On Monday, messaging app Line jumped into the MVNO market to stir up the competition with cut-rate offerings.