A 13-year deal, with £303million, with US giant Nike is due to expire in the summer of 2015.
Nike did have an exclusivity period in which to negotiate an extension.
But that has now expired. And United, whose commercial income has increased from £46.2million to £152million between 2003 - the first season of the Nike contract - and last year, a figure that will grow even more in 2014 given the massive new shirt sponsorship deal with Chevrolet, plus a multitude of other, smaller, 'partnerships', have opted to explore other options.
It is thought Adidas, Puma and Warrior have all expressed an interest in replacing Nike.
"We are currently in discussions with multiple parties regarding our global retail, apparel and product licensing business, starting in the 2015-16 season," said a club statement.
"This process is a routine practice used for our global and regional sponsorship deals and helps us maximize the value/structure of the club's partnerships."