T-Mobile USA, the No. 4 U.S. mobile provider, will sell a new unlimited data service option to its customers for a monthly fee in a direct challenge to its bigger rival Sprint Nextel, the only other big U.S. provider selling unlimited services.

The offer may also help T-Mobile USA, a unit of Deutsche Telekom, stand out from the biggest U.S. operators Verizon Wireless and AT&T Inc, which have both set caps on their customers monthly data usage.

Today T-Mobile USA, which has been losing customers to bigger and smaller rivals, limits its customers data speeds after they have downloaded a certain amount of data each month.

But under the new plan, which is being launched on Sept 5, T-Mobile USA will allow customers unlimited data usage without any curb on their service speeds.

The T-Mobile USA announcement came a day after smaller rival MetroPCS Communications announced a promotional offer under which it will also offer unlimited data usage without throttling back speeds.

Customers opting for T-Mobile USA's unlimited data usage will add a $20 payment on top of their existing $49.99 a month fee, which includes unlimited phone calls and texts and limited data.

This compares with the company's existing $74.99 plan, which comes with unlimited talking and texting and a limit of 5 gigabytes of data.