Companies must do more to open up their websites to people with disabilities or risk prosecution under equality laws, a trade body said.
Interactive Media in Retail Group (IMRG), the internet shopping industry body, said shops could lose four billion pounds in potential sales next year because many customers find their sites difficult to use.
More than a fifth of the population has trouble using Web sites, including people who are visually impaired or dyslexic, it said.
Under the Disability Discrimination Act, companies must ensure that everyone can access their sites. Most currently fall foul of the law, the IMRG said.
It wants retailers to help customers by adding a new piece of software to their sites that turns text into speech.
The group supports the new "toolbar" that sits on a shop's site and reads out words in a computerised voice, enlarges text and tinkers with colours to make the page clearer.
"It's the silver bullet," IMRG's Chief Executive James Roper told Reuters. "Put this little button on your site and suddenly you are legal."
IMRG and the device's developers at technology company the Hidden Differences Group say they can easily install it on a shop's site as a clickable "button".
Once activated, users can highlight text that will then be read out by a lifelike voice.
It also allows some content to be saved to a digital music player, to be listened to later.
Shops would be charged from 50 pounds per month for the service, while larger stores would pay more.
Although people with sight problems already use special software to browse Web sites, the new tool will put the onus on retailers rather than customers to make sites readable. It also means the costs are paid by the retailer.
"If it's available on the site, it's available to every user," said Jeremy Coulter, chief operations officer of the Hidden Differences Group.
The company's founder and Chief Executive Officer Kevin Thomson said clear, readable sites help firms stay on the right side of the law and boost sales by making shopping attractive.
"If you do something right for people with difficulties, everybody benefits," he told Reuters.
Internet shopping is booming. Shoppers spent 7.66 billion pounds online in the 10 weeks before Christmas, 53 percent more than in the same period in 2005.