Smartphone users have been warned they could face huge bills if they connect to the internet abroad.
The cost for using devices like the iPhone are currently unlimited when you access web services in Europe and around the world.
This is set to change in the 27 countries of the European Union from July 1, when new rules come into force which cap data download costs.
Consumer group Which? said it wants the EU limits extended to the rest of the world.
The costs apply if smartphone users check emails or surf the internet - functions which are normally included in unlimited download packages under most UK tariffs.
Introducing the new limits, the European Commission cited an example of a German traveller who last year clocked up a £40,500 bill after downloading a TV show while roaming in France.
Julia Feuell, from north London, also told of her surprise when her 17-year-old son racked up a bill of £590 during a visit to New Zealand.
From July 1, mobile phone users roaming across the EU will be protected by a default limit of 50 euros (£45) a month for data downloading.
Until then and elsewhere around the world, there are calls for mobile providers to warn customers before they breach their own tariffs.
Matt Bath of Which? said: "Mobile phone companies should voluntarily take these very sensible steps and apply them on a worldwide basis."
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