End of agreement between two firms means 4,000 BT hotspots could become unavailable to O2 customers.
O2 mobile phone customers will lose access to thousands of BT Openzone Wi-Fi hotspots from the beginning of July.
In an email sent to customers the mobile giant revealed the cuts, but said its own network of more than 8,000 Wi-Fi hotspots would still be available in shops and restaurant chains.
The change, which affects up to 4,000 locations, comes with the end of a four-year agreement between the two telecoms firms on 30 June. It means O2 will lose around a third of its current hotspot network, although it said its customers would still be able to use many of the affected sites.
Wi-Fi hotspots are popular among smartphone users as they offer internet connections that are usually quicker than 3G services. Users can also download emails in Wi-Fi zones without using their data allowance. About 6 million O2 customers currently use Wi-Fi sites.