Seven out of 10 telecom company websites fail to meet a basic level of usability for disabled users, says a report into web accessibility.
By law, firms must ensure that content is accessible to all users
The study by charity AbilityNet is part of its periodic evaluation of the sites of the top 10 companies in a industry.
Under the Disability Discrimination Act, companies must ensure that content is accessible to all users.
An estimated 12.5 million people in the UK currently have disabilities which make it hard for them to view websites.
All the sites reviewed were invited to make a public commitment to accessibility, but by the time of the report's publication only five - Vodafone, Kingston Communications, OneTel, Telewest and O2 - had chosen to do so.
AbilityNet rated each site out of a maximum of five stars using a mix of automatic and manual tests.
An award of three stars means that the site reaches a basic level of accessibility.
Out of the 10 sites investigated six achieved only two stars and the one belonging to the 3G operator, 3, achieved the lowest possible mark of one star.
A spokesperson from 3 said: "Accessibity for everyone is important to us and if that is the case that we were awarded one star, we will take that seriously."
She added that the web team was already looking at ways to enhance the website, and that they would take the findings into consideration and make the website more accessible.
The survey also showed reflected some positive developments.
Robin Christopherson, AbilityNet's web consultancy manager and author of the report, was "delighted" with the four-star results of two telecommunication companies, OneTel and Kingston Communications.
The report noted that a rating of four stars was very unusual, but that there is evidence that companies are starting to come round to designing accessible sites.
"We are now beginning to see examples of highly professional and accessible sites that prove incontrovertibly that an organisation's website can and should be accessible to the broadest audience possible," said Christopherson.
All of the companies which scored lower than three stars expressed their intent to take on board the report's criticisms.
A spokeswoman for T-Mobile, which received a two-star rating, told the BBC News website: "T-Mobile is committed to improving accessibility and recognises the findings of the AbilityNet report."
Four stars: OneTel, Kingston Communications
Three stars: BT
Two stars: T-Mobile, O2, Orange, Vodafone, Telewest, NTL
One star: 3
"The website is currently being re-designed and is scheduled to launch before the end of the year, taking into consideration the guidelines under the Disability Discrimination Act," she added.
NTL, which was also awarded a rating of two stars, said in a statement: "This is a matter which we take seriously and we aim to gain more than two stars in the future."
It also said that it would make changes to specific points raised by the report soon.
There are many ways in which websites can be inaccessible to disabled people.
Many blind or visually-impaired people, for example, use a screen reader - a piece of software which reads out loud a website.
But sites can contain levels of complexity in menus and style information which can make it difficult for these screen readers to pick out important content.
Flashing images and moving text can also trigger photosensitive epilepsy.
But there are less specific ways in which websites can be inaccessible.
Using small fonts, or designing the site in such a way that fonts cannot be resized by the user, makes the site difficult to read for people with even minor problems with their vision.
Studies carried out by the Disability Rights Commission have shown that sites which are accessible to disabled people score better in usability tests across the board.