The company now limits its customers to one gigabyte of downloaded data per day despite advertising that an advantage of broadband is "unlimited surfing".
Subscribers say the limit amounts to as little as two-and-a-half hours of use a day for a service that says it is "24/7".
"
It is no good having a Ferrari if you can only pop down to the shops in it
"
Kingsley Smith
Irate user
NTL says the move is necessary to help all of its customers get a consistent service.
Broadband has generally been sold on the basis that it is "always on" and is a much faster way of downloading pictures, videos and music.
But NTL's terms and conditions now limit downloading to a level consistent with "normal use".
It defines the one gigabyte limit as equivalent to "200 music tracks, 650 short videos, 10,000 pictures or around 100 large software programmes downloaded per day".
NTL's User Policy
It is not clear how NTL will enforce the download limit, or whether unhappy users will be able to terminate their contracts.
No-one from NTL's broadband section has yet been available for comment.
However, in a statement on NTL's authorised customer gripes site ntlhellworld.com, the company said the "vast majority" of users "will not be affected by this at all".
The statement continued: "ntl will only be contacting the small percentage of customers whose use of the service PERSISTENTLY exceeds normal levels, thereby potentially reducing the overall product performance for THE VAST MAJORITY OF other customers".
Businessman Kingsley Smith has set up a website to protest against the NTL's conditions.
He told BBC News Online: "I've had lots and lots of e-mails - I'm struggling to answer them all.
"It is disgusting the way they have slipped it in under our noses.
"They are taunting their customers."
Mr Smith said NTL had closed internal services because of cost-cutting which had forced users to download more.
"They have said this is the Ferrari of broadband services. But it is no good having a Ferrari if you can only pop down to the shops in it."
The Coventry resident said the situation was particularly irritating as in his area NTL had a monopoly on broadband.