By Brian Milligan
Business reporter, BBC News
Online shopping has risen in popularity in recent years
Thousands of internet retailers may be breaking laws that protect customers, claims a consumer advocacy group.
Which? is warning firms to obey the law, or risk being reported to the Office of Fair Trading (OFT) and face the threat of prosecution.
It said too many online sites were ignoring the rules.
Which? is warning retailers to make sure they comply with what are known as the Distance Selling regulations - such as time to return sold goods.
A Which? survey last month claimed that several well-known names, including Marks and Spencer and Game, didn't fully comply with regulations.
Both companies have since altered the wording on their websites, although both deny that they ever broke the law.
The Distance Selling regulations apply to anything purchased via the telephone, the internet or television.
"If you are an online store, it is your duty to stick with the legislation," said Matt Bath, the technology editor of Which?.
The biggest mistake made by retailers concerns the amount of time that customers have to return unwanted goods.
Customers have seven working days, from receiving the goods, in which to cancel the order.
They then have a reasonable amount of time to return them.
Providing cancellation was within that period, consumers are entitled to a full refund.
That means no site should charge an administration or re-stocking fee, unless the purchase was cancelled outside the seven-day period.
Retailers are also bound to refund any postage and packing fee on the original delivery.
However, depending on the conditions, the retailer may not be liable for the costs of sending the goods back.
Breaking the law
The Internet Media in Retail Group (IMRG) monitors the activities of 1,200 websites that are its members.
Andrew McClelland of the IMRG says it is probably "a fair assumption" that at least 1,000 sites may be breaking the law, though not IMRG members, he added.
He believes that few sites will be breaking the law knowingly.
"A lot of retailers new to the business, whilst not deliberately wanting to break the law, probably don't understand the requirements of the legislation," Mr McClelland said.
Which? is warning that it will now be hunting out other offenders.
"We will be keeping a firm eye on the big name stores and those smaller independents to make sure shoppers are getting a fair deal," said Matt Bath.
Any sites found to be breaking the Distance Selling regulations will be reported to the Office of Fair Trading, who could decide to prosecute.