Citigroup has signed a definitive agreement with American Express to purchase warehouse chain Costco’s co-brand credit card portfolio in the US for an undisclosed amount.

Under the agreement, Costco’s users with co-branded American Express cards will be transferred to Citi.

The transition is expected to close in mid-2016.

Citi Cards CEO Jud Linville said: "We are immensely pleased to have entered into an agreement to acquire the Costco portfolio and look forward to a long-term partnership with Costco and the opportunity to deliver value, convenience and seamless service to their 51 million loyal members across the country."

Currently, the bank is working with warehouse chain to offer a new value proposition to its cardholders.

Citi said that it does not expect the impact of the acquisition and conversion to be material to its earnings in 2016.