The watchdog said it has "reasonable grounds" to believe that Sky has contravened rules relating to landline and broadband services between 1 May 2015 and 31 July 2015, and has been investigating customer complaints for around one year now.
The complaints were made on the grounds that Sky was making it too difficult for customers to exit their contracts. None of the complaints related to Sky's TV services.
“We are incredibly proud to have the highest customer satisfaction levels in the industry, as ranked in the last Ofcom customer service report," said a Sky spokesperson.
"We have worked with Ofcom openly and constructively throughout their investigation so are very disappointed with this provisional decision. We will review the provisional assessment in detail and put forward our case before Ofcom makes its final decision.”
Sky now has one month to respond to Ofcom's provisional findings.
This update on the investigation comes just months after Ofcom revealed that Sky had received the fewest complaints about its broadband, mobile and TV offerings across the industry.