Online shoppers receive misleading information about their rights to return items, leading to widespread confusion, a consumer group has said.
Which? studied the websites of 46 retailers and supermarkets. It found that all but one had errors.
The only retailer to pass the test was hi-fi seller Richer Sounds. Ten others have now updated their information.
Shoppers have various rights to return unwanted and faulty goods bought online.
Under the Consumer Rights Act, you have an early right to reject goods that are unsatisfactory quality, unfit for purpose or not as described, in exchange for a full refund.
But this right is limited to 30 days from the date you took ownership of the goods. After the initial 30 days, you can't demand a full refund in the first instance, but you still have the right to a repair or replacement. Which? has a guide here.
When you buy goods online, you have additional rights to return them. This is because your decision may be based on a brief description or a photograph, so what you receive is not always quite what you had expected.
Under the Consumer Contracts Regulations, you are allowed to return an item bought online if you simply change your mind.
Exceptions include CDs, DVDs or software when the seal has been broken on the wrapping, perishable items and tailor-made or personalised items. They also include goods with a seal for health protection and hygiene reasons that has been broken.
You have the right to cancel at any time from the moment you place your online order, and up to 14 days from the day you receive your goods.
You need to notify the retailer. You then have a further 14 days from the date you notify the retailer of your cancellation to return the goods. Which? has a guide here.
The consumer group studied returns policies, frequently asked questions and terms and conditions on retailers' websites.
It said it found examples of shops unnecessarily pushing customers with faulty goods to their warranty agreements, failing to accept returns of faulty personalised items, or incorrectly stating different rules for different types of product.
In one case, Iceland's website stated it would not accept returns on unwanted items, or goods bought in error.
The retailer told Which? that its returns policy was "insufficiently detailed" and had now been changed.
Alex Neill, from Which?, said: "As a nation that is increasingly shopping online, it is important that trusted retailers do not mislead consumers about their rights.
"We will continue to challenge those that carry on confusing their customers."
A spokeswoman for the British Retail Consortium said consumers should be fully and correctly informed about their rights, but the law was "quite complicated" and efforts were being made to simplify terms and conditions.
"Many retailers go well beyond the law in giving consumers additional rights to exchange goods or get their money back if they have changed their mind," she added.