CBS and Viacom are finalizing their merger agreement, negotiating a share exchange ratio between 0.5960 and 0.5965, people familiar with the matter said.

The negotiations could finish as soon as Monday, the sources said, adding that the deal could be announced in the afternoon or Tuesday morning. The talks are ongoing, and nothing has been finalized, the people said.

An exchange ratio of about .59625 — the midpoint of the narrow range — would value Viacom at nearly $12 billion with an enterprise value of more than $20 billion. CBS will exchange its shares for each Viacom Class B share.

Viacom shares were down about 3.3% Monday morning, trading within the exchange ratio range. CBS shares were down less than 1%.

CBS and Viacom declined to comment.

A merger will add movie studio Paramount Pictures, cable networks such as Comedy Central, MTV, Nickelodeon and BET and assets including the streaming service Pluto TV and South Park Studios to CBS, which owns the eponymous broadcast network and other cable and media assets.

Shari Redstone, whose National Amusements is the principal owner of both companies, has advocated for a deal to give a combined company more financial heft to compete against media and tech heavyweights including Amazon and Apple for entertainment and sports rights, most notably the National Football League. CBS owns NFL broadcast rights until 2022 and "will do what is necessary" to renew them, according to CBS Sports chief Sean McManus.

CBS and Viacom nearly merged a year ago before a rift over who would run the combined company derailed discussions. Viacom also scrapped plans to merge with CBS in 2016.

The companies restarted merger talks in earnest this year after CBS replaced six members of its board of directors and its CEO, Les Moonves, in September. Moonves stepped down as chairman and CEO amid sexual misconduct allegations. Discussions have dragged on throughout the year as the new CBS board has become more comfortable with the idea of merging with Viacom rather than pursuing other paths.

This story is developing.