CBS and Viacom likely won't reach a merger agreement in time for CBS' earnings release on Thursday, according to sources familiar with the situation. The companies originally hoped to have a deal by then.

The companies are still negotiating over the exchange ratio, sources said. A deal is expected in the coming days. CBS and Viacom do not plan to answer questions related to the merger on Thursday's earnings call.

CBS and Viacom declined to comment.

Shari Redstone, vice chairwoman of the board at CBS and Viacom, 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. The boards had agreed upon an exchange ratio of 0.6135 CBS share for every Viacom Class B share at the time. Viacom also scrapped plans to merge with CBS in 2016.

The merger is expected to see Viacom CEO Bob Bakish take over as CEO of the combined company, sources said. Joe Ianniello replaced ousted CEO Leslie Moonves last year. Ianniello is expected to remain at CBS in a new role, sources said.