Curve, a London-based startup that lets users replace multiple cards with a single piece of plastic and a mobile app, has been forced to kill off support for American Express.
In a Q&A on its site, Curve says that AmEx approached it a few weeks ago "with their concerns about ensuring a seamless customer support process for its Members when using its Card with Curve".
AmEx cards will stop working with Curve at the end of the month, although the startup says that it is still in talks with the US giant and hopes a reversal could happen sometime soon.
AmEx's decision could prove a costly blow to Curve, which only launched in February after securing a $2 million seed investment from high profile investors such as Taavet Hinrikus of TransferWise and Ricky Knox of challenger bank Tandem.
Although built on the MasterCard Network, one of the card's major selling points was its ability to let users recharge transactions to their AmEx accounts. This meant that they could use AmEx at places where it is not usually accepted.
In addition, the card could be used for ATM withdrawals that were treated as purchases so that users could earn air miles, points out the FT, although an 'acceptable use' policy was introduced in April.
In light of the latest developments, Curve is offering a refund to people who have bought its card, which ships for £35.