The investigation covered 28 European countries
More than half of websites selling electronic goods and tested in an EU investigation were breaking European laws aimed at protecting consumers.
The analysis of 369 websites selling mobiles, DVD players and games consoles in 28 European countries found that 203 of them held misleading information.
The biggest failure surrounded the right to return a product bought on the internet within seven days.
Any websites which continue to break the law face fines.
"We know from the level of complaints coming into European Consumer Centres that this is a real problem area for consumers," said EU consumer commissioner Meglena Kuneva.
"We discovered that more than half of the retailers selling online electronic goods are letting consumers down."
Authorities, such as trading standards departments, carried out the investigation in May. They were checking to see if the websites followed rules on providing clear information about the trader, the product, the price, and customers' rights.
Some 369 websites - across 26 EU member states (all members except Slovakia) as well as Norway and Iceland - were checked as they sold electronic goods including digital cameras, mobile phones, personal music players, DVD players, computer equipment and games consoles.
Two hundred of the sites were chosen because they were the biggest in the EU and another 100 were checked because they had been the subject of previous consumer complaints.
Of the 203 cases facing further investigation:
- Two-thirds (66%) failed to adequately explain that consumers had seven days to return a product bought over distance for a full refund and without giving a reason. Others failed to explain the right to have a faulty product repaired or replaced for at least two years after sale
- Details about extra delivery charges were missing or difficult to find on the website in 45% of cases
- A third (33%) did not fully outline the trader's name, address or email details so they could not be contacted if there was a problem.
All of these traders will now be contacted by the authorities and asked to clarify the position or correct the problems identified in the investigation.
Meglena Kuneva is the EU consumer commissioner
Any website that fails to make corrections could face warning letters and then enforcement action. If this was ignored the operators could be prosecuted and face fines.
"This is a Europe-wide problem which needs a European solution. There is a lot of work to be done in the months ahead to clean up this sector, Europe's consumers deserve better," said Ms Kuneva.
Every website checked in Cyprus and Hungary during the sweep was found to require further investigation. Six of 14 websites checked in the UK revealed irregularities.
Only Iceland, Norway and Latvia have published a list of the websites that will face further investigation.
About one in four consumers across the EU who has ever bought anything on the internet bought an electronic product, according to the European Commission. The market is valued at an estimated 6.8bn euros (£5.9bn).
Some 34% of complaints about online shopping in 2007 featured the sale of electronic equipment.
David Smith, director of operations at UK online retailers' trade body IMRG, said: "It is disappointing that some companies are failing to apply basic standards."
He said that IMRG oversaw a code of conduct that stipulated that traders should be aware of, and adhere to, the rules. However, he added that the picture was blurred by the fact that different rules tended to apply across different European countries.