The telecoms regulator has also published a consumer guide to using mobile phones abroad.
International roaming is a service that allows mobile phone users to make and receive calls and text messages while travelling abroad. Each UK mobile operator has established agreements with overseas networks that allow its customers to use their mobile phones in many countries around the world, providing their handset is compatible with those networks.
But Ofcom says that international roaming agreements are complex and wholesale charges between network operators can vary significantly. This has led to confusion among users over the wide range of prices they might pay. Although some operators now offer simpler, unified retail tariffs, some consumers still incur substantial mobile phone bills as a result of international roaming charges.
The watchdog has therefore started an assessment of the UK market, alongside similar investigations undertaken by other national regulatory authorities in Europe.
Ofcom has also published a consumer guide, providing advice on how to reduce costs and setting out the prices that most UK subscribers are likely to pay when making or receiving calls or text messages overseas.