Citizens Advice said that travellers who did not buy a "bolt-on" - allowing for a set amount of data use - could see higher bills.
The other option was simply for holidaymakers to turn off data roaming on their devices while travelling.
The charity is calling for a price cap to ensure nobody is caught out.
Among the cases it has seen are military personnel being billed for thousands of pounds by their mobile operator while deployed abroad, as well as holidaymakers facing high charges after using maps, uploading photos or downloading albums.
In June, it was announced that data roaming charges would be abolished within the European Union by June 2017. The ban will be preceded by a 14-month interim period, in which companies can still add surcharges - but at a reduced rate.
Holidaymakers travelling outside the EU risk paying mobile bills up to 80 times higher than they need to, owing to roaming charges, a charity has warned.