Action has been taken against four websites who charged for European health insurance (EHIC) cards that are free from the NHS.
Typically the websites had charged £10 for processing applications that people could have made themselves.
While not an illegal practice, the sites were deemed to have misled customers into believing they were providing an official service.
The cards give UK holidaymakers free or cheap state health care in the EU.
Three of the websites have agreed to stop "deceptive selling practices" and two others have stopped trading.
'While it is not unlawful to charge money for a reviewing and forwarding service, traders must be clear about the product or service they are offering, and not trick consumers into parting with money for services they don't want," said Heather Clayton of the OFT.
"People seeking to obtain government funded services such as an EHIC should check carefully that they are using an official government website."
UK citizens are officially encouraged to get an EHIC card - formerly known as an E111 card - before going on holiday and they are free from the NHS's own website.
In June, the BBC's Money Box programme highlighted the issue of firms trading on the internet and charging for access to free government information or services.
Three websites have now signed an agreement not to mislead their customers about the heath cards.
They are www.ehic.org, www.thejwsgroup.com/ehicservice, and www.ehic.uk.com.
All three websites are currently unavailable.
Another, www.ehiconline.com, has been shut by its domain name registrar after the OFT complained, while the fifth, www.e111-online.com, has shut itself down.
The OFT said it started investigating the websites in March after complaints were made to the Department of Health and to Consumer Direct.
The OFT found that the sites were using a variety of tricks to fool customers into thinking they were paying for an EHIC card via an official website.
The sites, the OFT said, used the EHIC brand, used domain names that made them sound official, and they also failed to mention they were not, in fact, official providers of EHICs.