Shares in Carphone Warehouse have fallen by 12% after Vodafone said the retailer would no longer sell its mobile phone contracts.
Vodafone is giving rival Phones4U an exclusive contract to sell monthly phone subscriptions, an arrangement it currently shares with Carphone.
Carphone, which will still sell pre-pay connections, said the change would not harm its financial position.
Vodafone said the move would reduce the amount it pays in commissions.
The change in arrangements will come into effect on 30 November.
Carphone Warehouse has a larger retail presence than Phones4U and the deal is a major boost for the latter, recently sold by its founder John Caudwell to two private equity firms for £1.46bn. About 40% of Vodafone's network connections are sold on a contract basis, typically over a year to 18 months