Websites that show bargain prices for flights, hotels and other travel bookings, are not giving customers accurate information, say European consumer protection authorities.
The first price shown was often much lower than the final price, they said.
Some offers that look too good to be true, are - because when you click to buy they aren't available.
"These websites are misleading consumers," said Vera Jourova, the EU commissioner responsible.
The Consumer Protection Cooperation body said the 235 websites that they had identified would be required to correct the problems.
If websites failed to comply, national authorities could pursue legal proceedings, it said.
"The companies concerned need to respect the European consumer rules, just like a travel agent would," said Ms Jourova, the EU Commissioner for Justice, Consumers and Gender Equality.
- In one third of cases the first price shown was not the same as the final price to pay.
- In one fifth of cases promotional offers were not really available.
- In nearly one third of cases the way the total price was calculated was not clear.
- In one quarter of cases prompts on scarcity (eg "only 2 left") only applied to availability on that particularly website, which wasn't made clear.
The CPC screened the sector in October 2016, covering 28 European countries. It checked a total of 352 sites, including ones offering to book accommodation, transport tickets and car rental. Some were price comparison websites.
It also found that over a fifth of the sites it looked at presented consumer reviews in an unclear way, sometimes throwing doubt over their truthfulness.