The cause of the problem is currently unknown, but Telewest engineers are understood to be treating it as a top priority and are hoping to resolve the situation this evening.
The three services affected by the outage include Blueyonder's Personal Web Page (PWP) servers, which carry all its subscribers' Web sites; SelfCare, which allows users to manage their Web sites, add and remove MAC addresses and monitor traffic; and the email servers.
The cable firm's technical support team warned late on Tuesday afternoon that: "All blueyonder customers will currently be experiencing problems accessing their email, Webmail, the selfcare tools on the blueyonder Web site and their personal Web page."
"This issue is being investigated as a matter of the highest priority and we hope to resolve shortly," Telewest added.
According to its users, Telewest offers a relatively robust service. However, earlier this month the company did admit that this summer's virus attacks drove some broadband users away.