From 1 July customers of O2 will no longer have access to BT Openzone wi-fi hotspots around the UK, when a four-year deal between the two firms ends.
In an email to customers, O2 says its own network of more than 8,000 wi-fi hotspots will still be available in various shops and restaurant chains.
BT Openzone has five million hotspots but O2 says that only 4,200 of them were ever shared with its customers.
It means O2 will lose around one third of its current wi-fi hotspot network.
The change is the result of a four-year agreement between the two telecoms firms coming to an end on 30 June.
"From 1 July 2013, we will no longer offer access to BT Openzone wi-fi hotspots to our customers," said O2 in a statement.
"We'll continue to extend the reach and scale of O2 wi-fi through exciting partnerships with venues including O2 shops, restaurants, retail outlets and outdoor and indoor locations across the UK."
The firm says it has six million wi-fi customers.
The end of the deal with BT Openzone follows disappointment for O2 in the 4G spectrum auction held in February, Kester Mann, senior analyst at CCS Insight, told the BBC.
"The reduction in the number of hotspots is a blow to O2 customers," he said.
"At the recent 4G auction, O2 failed to secure frequencies at 2.6GHz. This may mean that it will struggle to meet the growing data-needs of its customers, particularly in high-density areas.
"A focus on alternative technologies such as wi-fi may have given the operator the opportunity to address this potential shortfall," Mr Mann added.
"This is particularly important given that its 4G licence mandates a minimum indoor coverage requirement of 98% of the UK population by the end of 2017."