Using the internet on your mobile while abroad can end up costing more than the holiday itself, Citizens Advice is warning.
The charity said people travelling outside the EU can face bills of thousands of pounds due to high charges and because they do not realise how quickly costs can rise or even that they are using roaming data.
It is urging the Government and providers to work on a voluntary agreement to stop shock bills, which would include more warnings for consumers about costs and a voluntary price cap.
Within the EU, the highest per megabyte roaming rate is capped at 17p, but outside the EU this does not apply - and researchers found it can cost up to £12.50, the charity warned.
Citizens Advice found that, for example, watching an episode of EastEnders could cost up to £1,360 in Turkey, compared with a maximum of £32 inside the EU.
Some of the people the charity has helped include military personnel who were billed for thousands of pounds after leaving their data roaming on while deployed abroad and a holidaymaker who turned on data roaming to download an album while in Egypt and was charged £1,500.
Consumers pay different rates depending on which provider their contract is with, where they travel, and whether they have bought a “bolt-on” which gives them a set amount of data to use abroad for a fixed cost.