From February 15, Debenhams, one of the founding partners when Nectar launched in 2002, will no longer issue or redeem Nectar points.
It is understood that Debenhams will now focus its loyalty marketing solely on its own store card holders, and customers signed up to its Mastercard points accumulation programme.
The retailer has denied having plans to launch a new loyalty scheme or joining one of the rivals to Nectar.
Loyalty Management Group, which owns Nectar and was acquired by Aeroplan last year, said it was disappointed Debenhams would not be renewing its loyalty association with Nectar when it expires next week, and was now searching for a replacement retailer to join its programme.
Nectar's other partners include Sainsbury's, BP, Ford and the AA.
A spokeswoman for LMG told Brand Republic that the group had been continuing to sign up partners to its Nectar eStores programme over the last few months, and would be focusing on securing a partner "with an online offering on the department store front".
In a statement, Rupert Duchesne, chief executive officer of Aeroplan, said: "Notwithstanding the loss of Debenhams, Aeroplan's financial guidance for LMG issued at the time of the acquisition remains unchanged."