Old Mutual is selling 9.6% of Quilter, which is being hived off as the final part of a break-up plan that will see the South African group broken into four parts.
READ: Old Mutual to sell Latin American businesses to Singapore-based holding company CMIG International for undisclosed sum
Quilter has over 900,000 affluent customers.
A final offer price and the number of shares to be sold in the Quilter offer are set to be announced on June 25
Old Mutual added that another part of its break-up - the sale of its "single strategy" asset management business - is progressing in line with plans.