AT&T has followed Comcast's lead in announcing that it will introduce data caps for its internet customers.

The company will this month begin capping users of its slowest 768Kbit/s DSL service at 20GB per month. The limit increases with broadband speeds, reaching 150GB per month at the 10Mbit/s level.

"AT&T plans to initiate a broadband internet access usage trial in Reno, Nevada beginning in November," said the company in a Federal Communications Commission filing.

"AT&T will be providing written notice to customers involved in the trial explaining that their broadband service will be subject to a certain monthly usage tier for the total amount of data they may send and receive, as well as a per-gigabyte charge in the event they exceed the usage tier."

The company estimates that five per cent of users on its networks account for 50 per cent of the traffic.

The caps will initially be restricted to customers in Reno, and AT&T will gauge the reaction before deciding on whether to roll out the policy nationally.

Customers will be able to track their monthly data usage on AT&T's web site, and will be warned when they reach 80 per cent of capacity. Charges of $1 per gigabyte will apply over the capped limit.

Other ISPs are also reportedly considering similar bandwidth caps. Only Verizon has declared that it has no plans for such a policy.