The Wall Street Journal reported on the talks late Monday, citing people familiar with the matter.
The deal for exclusive talks through late July puts Sprint's merger talks with T-Mobile US on hold. The cable companies could take an equity stake in Sprint.
The talks could include the possibility of the cable companies jointly buying Sprint.
A Comcast spokesperson declined to comment.
Sprint and Charter declined to comment when contacted by CNBC.
T-Mobile US didn't immediately return CNBC's emailed requests for comment, which were sent outside office hours.
Disclosure: Comcast is the owner of NBCUniversal, the parent company of CNBC and CNBC.com.