Which? has alerted UK smartphone owners over the potential financial impact of using a smartphone to surf the internet whilst overseas.
The consumer magazine said it was prompted to raise awareness of the issue after being contacted by several UK citizens who had inadvertently run up high charges for mobile use.
In July, new EU legislation will impose limitations on operators relating to charging customers when they access data networks abroad.
However, until this comes into force, people are still in danger of being hit with an unexpectedly high bill.
In order to avoid incurring charges, Which? is advising anyone who has a push email service active on their smartphone to turn this off until they return home.
Push email automatically sends received messages on to the user when they are received and can silently cost a good deal of money if left active.
Matt Bath of Which? said: "[We are] calling on mobile phone operators for transparency on using phones abroad and especially for those who use the internet abroad.
"We've spoken to people who've racked up bills of tens of thousands of pounds."
Mr Bath added that it cost an average of £8 per megabyte for data use abroad and in some instances a quick GPS search had cost the user upwards of £70.
People streaming TV shows to their smartphones while out of the country are another group running the risk of high data charges.