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Unless online retailers rectify the "staggering" situation they could be forced out of business, warned law firm Browne Jacobson, which conducted the study.
The research, which surveyed the websites of the top 50 UKretailers to analyse their compliance with legal requirements for selling to customers over the internet, found that consumer rights were being infringed on 24 of the sites.
While the top 10 retailers complied with all the necessary legislation, a number of top-50 companies were less compliant, the study found.
For example, one website failed to contain an appropriate data protection consent form - an oversight that could be punished with a £5,000 fine from the EU. Another website informed users that their personal details would be passed onto other firms unless they sent an e-mail opting out - a direct contravention of the Data Protection Act.
Even more damning was the revelation that eight of the sites failed to meet any of the requirements set out under the E-Commerce Regulations, Data Protection Act or the Distance Selling Regulations.
Simon White, internet law specialist at Browne Jacobson, said the lack of compliance could have serious consequences for retailers.
"With so much business being done over the internet, it is staggering that leading retail chains and high street brands should fall foul of such basic requirements, particularly given that its is so straightforward to meet the letter and the spirit of the law," he said. "Unless they brush up their act, the operators of these websites could be forced out of business."