In excess of a third of the most popular UK online retailers have been told to change their websites by the Office of Fair Trading (OFT).
The OFT found that 62 out of the 156 sites checked in total might not be in full compliance with the consumer protection laws currently in place.
It checked the 100 most-used online retailers in the UK, as well as many online clothing sites.
In particular, the regulator found that some retailers were adding compulsory charges without warning customers when they checked out.
It also stated that some sites placed restrictions on refunds which were unreasonable, such as wrongly informing customers that they could only return items that were still in their original condition or packaging.
By implementing such a restriction, the OFT stated, the buyer's right to inspect a product could be breached.
Retailers breaching the consumer protection laws have been written to by OFT, who want to see all sites complying by Christmas (2012).
Commenting on its findings, senior director of the goods and consumer group at the OFT, Cavendish Elithorn, stated: "The OFT recognises that most businesses want to play fair with their customers and to comply with the law.
"We encourage all online retailers to check their websites so customers can be confident their rights are being respected when they shop online."
This isn't the first time large companies have been reprimanded for their online activities.
Back in June, Nike was warned about its Internet marketing techniques after England footballers Wayne Rooney and Jack Wilshere tweeted marketing messages on behalf of the clothing giant, without clearly flagging the messages as advertisements.