The consumers' association Which? is warning that people going abroad with smartphones can still face huge bills if they connect to the internet.
By 1 July, new rules will come into force in the European Union which will cap bills for downloading data.
But, until then, people travelling in Europe could face unlimited bills.
And anyone visiting non-EU countries, like Turkey, the US or the Caribbean, will continue to have no limits on their internet usage.
Bill shock
If you use your phone in the UK to connect to the internet, for example to check emails or go on Facebook, you don't usually need to worry about the bill - most home tariffs include unlimited downloads.
But, if you take a smartphone, like an iPhone, on your travels, it can have expensive consequences.
One German man was reported to have been charged £41,000 after downloading a television programme onto his phone.Julia Feuell, from north London, also got a shock after a visit to New Zealand. Her 17 year-old son racked up a bill of £590.
"It was a telephone bill that I'd never seen in my life before. It was a great shock to Alex, who's an apprentice mechanic."
However, the phone company concerned eventually agreed to halve the bill.
According to research by Which?, people using smartphones abroad can pay up to £8 for every megabyte downloaded. That's the equivalent of one email with a photo attachment.
But anyone who downloads videos or films can expect to pay considerably more.
According to Which?, a ten minute video clip and five music tracks could cost as much as £200.
New EU rules
If you travel within the 27 countries of the European Union, or Switzerland, you will soon be protected by new rules to limit bills for data downloading.
From the 1 July this year there will be a default limit of 50 euros (£45) a month.
Until then, it's up to consumers to get in touch with their phone company to get that, or a different limit, applied to their account.
Users will receive a warning when they are approaching 80% of their limit, and will then be cut off once the limit is reached. But travellers to the rest of the world will receive no such protection.
Which? would like the EU data download limits extended to the rest of the world.
But this would have to be done by the phone companies themselves, as there is no regulatory body that has global reach.
"Mobile phone companies should voluntarily take these very sensible steps, and apply them on a worldwide basis," says Matt Bath of Which?.
But the GSM Association, which represents global mobile phone companies, disagrees.
"Europe is a very unique market. We would not advocate copycat regulation for other territories," a spokesperson told.
It also says it is trying to drive down bills by other means, and points out that the price of mobile services has already fallen by a third in the last five years.
Agree limits
Some owners of smartphones are unaware that their phones roam the internet whenever they are switched on.
With so-called "push email" programmes, that means you will be charged whenever someone sends you an email.
Equally, if you use an application to search for a nearby restaurant, or go onto Google maps for local directions, you are downloading data.
Which? advises users to go into their settings and simply turn off the data roaming facility.
Users should, in any case, be warned about this whenever they arrive in a foreign country and their mobile operator is substituted by another.
Otherwise, if you are travelling to Europe before the 1 July, you should get in touch with your phone company to agree a limit on data downloads.
Those who want an allowance which is larger than 50 euros should also contact their phone company, to get the limit raised.
