GMG Radio, which includes stations such as Real Radio and Smooth Radio networks, has been sold to Global Radio for an estimated £50m.
Stuart Taylor, who has led GMG Radio since 2009, will leave the company as part of the sale.
Global, who already handles the national advertising contract for GMG, may have to deal with competition issues as part of the deal in areas such as Cardiff and the Midlands.
Andrew Miller, GMG chief executive, said in an email to staff: "The purpose of GMG's portfolio is to support and develop our journalism.
"This means that we are sometimes faced with the difficult choice between retaining a business that has performed well, as with this case, or deciding that a disposal at an attractive value represents the best way of discharging our responsibilities."
The sale was predicted a fortnight ago by the Guardian, which is also owned by GMG.