Telewest is a broadband company that offers cable, television, telephone and Internet access.
BlueYonder spokesperson Alexandra Legg told Macworld: "We're up to speed in supporting OS X. Our technical support team is now fully trained on OS X and we're committed to providing first-class technical support to our Mac users."
BlueYonder claims it was the first UK ISP to offer a full broadband service to Mac users in February.
BlueYonder's announcement comes hot on the heels of AOL's US release of Mac OS X, its Internet access software.
Training pledge BT Openworld supports OS X but its staff have yet to receive technical training, and are unable to provide technical support. The company says it plans to provide training "soon".
MacUnlimited was the first Mac ISP to support OS X, according to executive director, Farhad Alaaldin: "Initially, people were unsure how to set up with Mac OS X, but our Web site provides a step-by-step guide for this.
"We have found more and more people are using Mac OS X, because of the enhancement in the OS and its better user-interface. We are delighted with Mac OS X."
Meanwhile, although Demon's dial-up Internet service is compatible with Mac OS 8.5 and above, it does not support ADSL on OS X.