Many online retailers are breaking disability laws by having poorly designed websites, according to internet usability specialists Webcredible.

A survey of the top 20 online retailers in the UK found that over a third were breaking the Disability Discrimination Act by having sites that are too difficult for some disabled people to use.

The main problems are small text size and final checkout pages which are too complex.

"Some of the best performers let themselves down through sloppiness," said Trenton Moss, director of Webcredible.

"For example, accessibility guidelines are adhered to on most pages but then not on other key sections.

"Surprisingly, the checkout processes were generally poor. There is no point in a site being accessible if there is a 'show-stopper' in the checkout and users cannot continue."

The company rated sites in 20 categories covered by the Disability Discrimination Act, awarding between 1-5 points in each area.

Electrical retailer Currys picked up the worst score in the survey at just 34 points, while HMV nearly topped the poll with 75 points.

Research by Ofcom suggests that disabled computer users under 45 are online more than the national average, and that there are 8.6 million disabled people of working age in the UK with a combined spending power of £80bn.

"Disabled web users of all ages are becoming increasingly savvy and adept at using the internet," said Moss.

"Increasing the accessibility of any e-commerce website will lead to a more commercially successful site and result in increased profits."