More than a third of the UK's top online retailers could be breaking consumer laws, the Office of Fair Trading (OFT) has said.
A check of 156 popular websites suggested that 62 may not be fully complying with consumer protection law.
The OFT found unreasonable restrictions on refunds and compulsory charges being added without prior warning at the online checkout.
It has asked the retailers to change their websites before Christmas.
In this year's sweep, the biggest so far, the regulator checked the sites of the 100 most-used online retailers in the UK, as well as the most popular clothing sites.
Among the most common problems found by the OFT were retailers wrongly telling their customers that returned goods must be in their original packaging or in their original condition.
The OFT explained that this could be in breach of the buyer's right to inspect or assess a product.
Nearly two-thirds of all the retailers checked failed to provide an email contact address, as opposed to a web contact, which is a breach of the E-Commerce regulations.
Of the 60% of sites that notified buyers there would be compulsory charges, such as delivery, in addition to the up-front price, 24% of those then added extra, unexpected, charges as well.
These included card charges, and booking and luggage fees.
Overall though, the OFT's survey found that most online retail websites complied with their other obligations under the Distance Selling Regulations.
Consumers shopping online, by phone or mail order, are covered by these regulations in addition to their regular rights, which ensure that all goods bought are fit for purpose, for example.
The distance sales rules state that goods should be delivered within 30 days and that shoppers should be given a 7-day cooling-off period in which they can cancel an order and get a full refund. This period is longer for financial products.
"The OFT recognises that most businesses want to play fair with their customers and to comply with the law," said Cavendish Elithorn, a senior director at the OFT.
"We encourage all online retailers to check their websites so customers can be confident their rights are being respected when they shop online."
It has now written to all of those who may be misleading consumers.
Those who do not change their sites to comply with the law could be taken to court and face fines, the OFT said, although in past cases, most have altered their sites without it going this far.
Online shopping has boomed in the past decade and the OFT has regularly warned retailers to make sure they are not ignoring their legal obligations.