RBS was required to divest 308 branches as a condition of the state assistance it receives.
The consortium will put £600m of investment into the branches, fronting £330m while they are lent £270m by RBS.
RBSG's Markets division is providing a £270m secured financing package to the Investors for the investment.
RBSG chairman Sir Philip Hampton commented on the pre-float investment, saying that RBSG are "delighted to be working in partnership with these investors", with today's announcement providing "more certainty for our customers and employees ahead of a flotation."
Group finance director Bruce Van Saun said that deal was a "very competitive process" and that RBS believes the transaction demonstrates that "Williams & Glyn's is a viable and attractive business which will be positioned as a strong, customer-focused challenger bank."