Sony said Tuesday its board had "unanimously" decided to reject a US hedge fund's proposal to spin off part of its profitable entertainment arm.

"The Sony board of directors has unanimously concluded that continuing to own 100 percent of our entertainment business is the best path forward and is integral to Sony's strategy," Sony president Kazuo Hirai said in a letter to US billionaire Daniel Loeb.

"We ... will continue to give full consideration to any constructive feedback from our valued shareholders," Hirai said, promising to maintain "a productive relationship with you and welcome an ongoing dialogue".

Loeb, who says his hedge fund Third Point has amassed the largest stake in Sony, called on the firm's executives to spin off up to 20 percent of the entertainment unit, which includes a music label and a Hollywood movie studio.

Loeb had argued the spin-off would make the entertainment unit's managers more accountable and help improve profitability.