Mobile networks must tell people if they are going to be charged for using their phone abroad under new plans from the UK regulator.
Ofcom wants people to be alerted about potential fees for roaming – using minutes, texts or data outside the UK.
It says one in five people currently do not realise they face such fees when travelling.
Under the plans, customers must also be told about any action they can take to limit roaming-related charges.
Consumer groups say they are currently costing British phone users more than half a billion pounds a year.