O2 saw a 40,700 reduction in its home broadband customer base during the first quarter of 2013, it has been reported.
The pace of customer attrition accelerated during the first three months of the year, with the total number of subscribers falling from 560,100 to 519,400.
This came after a net 19,400 home broadband subscribers left the company in Q4 2012, and the total fell by 22,500 in Q3 2012.
O2's fixed-line customer base has been contracting since the final quarter of 2012, when the firm reached a peak of 671,600 subscribers, reportsISPreview.co.uk.
Earlier this year, it was announced that Sky had agreed a deal to take over O2's home broadband service - and that of sister company Be Broadband.
Sky is reported to be paying at least £180 million for the services, which will see O2 and Be Broadband's fixed-line customers migrate over to Sky.
Ronan Dunne, Chief Executive of O2's parent company Telefónica UK, claimed the sale of its home broadband service should help safeguard the customer experience.
He noted that Sky offers "great value" and totally unlimited home broadband.
"As we focus on delivering best-in-class mobile connectivity, including next generation 4G services, we believe this agreement is the best way of helping our customers get the highest quality home broadband experience from a leading organisation in the market," Mr Dunne added.