Travellers to the Rio Olympics have been warned to check their mobile data caps and stick to them or risk racking up bills of thousands of pounds.
All mobile operators have to apply a data cut-off limit of 50 euros - around £40 - per month for customers wherever they travel in the world, but there is the choice to opt out.
An average of 8MB of mobile data - enough for just 23 social media photo updates or two minutes of streaming - would cost £40 outside of a bundle in Brazil. This would mean those travellers tempted to opt out of the cut-off limit could "easily" amass a bill of £197 a day, energy provider comparison firm uSwitch said.
But costs for calls, voicemail retrieval and text messages are still uncapped. USwitch calculated UK mobile users could spend more than £47 a day on minutes and texts alone based on making three five-minute phone calls home, receiving two five-minute phone calls, listening to a two-minute voicemail message and sending 10 texts.
Most operators offer substantial levels of protection against mobile bill shock abroad, but uSwitch found just 18% of us check with our networks for cheaper roaming deals, while 16% think using a mobile outside the EU is free and 49% are not sure.
Ernest Doku, mobiles spokesman at uSwitch, said: "Never has there been more urgency for networks to improve rates for overseas mobile usage, particularly now lower roaming prices inside the EU potentially hang in the balance following the UK's decision to leave.
"Right now, the £40 cap is all well and good in the EU - where roaming prices are currently lower - but potentially astronomical roaming costs further afield means people are tempted to opt out, making them more vulnerable to bill shock."
:: Censuswide surveyed 2,171 UK telecoms customers between April 13-15.