AT&T Mobility LLC has been hit with a $100 million fine for offering consumers “unlimited” data, but then slowing their Internet speeds after they hit a certain amount.

The Federal Communications Commission said Wednesday that the company misled consumers into buying plans they believed would give them unlimited ability to send and receive data, including Web browsing, GPS navigation and streaming videos. But the FCC said that once the consumer reaceched a certain level, that data would be slowed down significantly, at speeds lower than advertised.

AT&T said it will “vigorously dispute” the allegations.

“The FCC has specifically identified this practice as a legitimate and reasonable way to manage network resources for the benefit of all customers, and has known for years that all of the major carriers use it,” the company said in a statement released to reporters. “We have been fully transparent with our customers, providing notice in multiple ways and going well beyond the FCC’s disclosure requirements.”