Holidaymakers leaving the EU - including football fans going to the World Cup - are being warned about the risks of running up huge mobile phone fees.
UK holidaymakers in the EU benefit from caps on the cost of making calls, sending texts and downloading data.
However, watchdog Consumer Focus is warning that such limits to not apply to those travelling further afield.
Football fans uploading 10 photos at a game could be charged up to £80.
Using a phone in the UK to connect to the internet usually has no effect on the bill as most home tariffs include unlimited downloads.
But, taking a smartphone, such as an iPhone, on your travels can have expensive consequences.
The watchdog said that the cost of using mobile internet - increasingly popular as people visit social networking sites on smartphones - can be up to £8 for every megabyte downloaded. That is the equivalent of sending one e-mail with a photo attachment.
Charges to make calls ranged from 80p to £1.50 a minute, and texts cost from 25p to 50p, Consumer Focus said.
"England fans need to be on the ball to make sure they do not get caught out by hundreds of pounds of hidden mobile phone costs at the World Cup," said Nick Hutton, of Consumer Focus.
"We want to see mobile phone firms play fair by customers travelling abroad and cut these extremely high fees.
"In the meantime consumers should check their network rates before they fly and look into the add-ons or bundles on offer, which could save them much needed cash."
Holidaymakers travelling in the EU have a maximum tariff of 43 cents (37p) a minute when making a call, 19 cents (16p) a minute when receiving a call, 11 cents (9p) a minute to send a text, and, from July, 50 euros (£42.50) for data roaming - unless an alternative limit is set.
Users receive a warning when they are approaching 80% of their limit, and are cut off once the limit is reached.
Consumer groups want the limits to be extended to the rest of the world.
Matthew Wheeler, of price comparison website Uswitch, said travellers should do their homework before leaving, if they intended to use their mobile. Tips included:
- Switch off data roaming on the mobile settings
- Ask the provider to cap data roaming charges to an affordable level
- Consider buying a local or international SIM card
- Consider texting, rather than calling, and share one mobile among friends.
"While some networks are offering special deals, roaming abroad is still far from cheap," he said.
Financial information service Moneyfacts also warned travellers to take care when changing money for a trip.
Exchange rates can be high at airports, as can some services overseas. Commission-free changing can have worse exchange rates.
Credit card providers often charge for the purchases and cash transactions every time a card is used.
The same might be true of debit card usage, and some charge a fee to take money out of a cash machine - unlike in the UK.
"Customers taking out cash need to be aware of minimum charges. If you withdraw lots of small amounts you will be hardest hit, so it may be worth planning a few days ahead for your financial requirements," said Michelle Slade, of Moneyfacts.