The trial, which is running across nine stores in South Wales, offered all customers, including its chargecard holders and non-storecard holders, an M&S-branded credit card that rewards shoppers each time they use the card.
The credit card on trial is available as Visa and Mastercard and shoppers are offered one point for every £1 they spend in M&S and half a point for every pound they spend in other stores.
Points can be redeemed in exchange for vouchers, which can be spent on M&S products.
The card is designed to help boost its relationship with customers and build on its chargecard, for which it has 5m customers. However, its chargecard was badly hit when the store introduced payment by credit card in 2000.
The launch will also see M&S join rival retailers such as Tesco and Sainsbury's, to compete with major banks such as NatWest in the competitive credit card market.
The card is being launched by M&S Financial Services arm and is expected to double the store's profits from its financial services activities.
M&S is expected to report results of the trial in Wales today, as well as full-year pre-tax profits of £710m, up from £646m last time.
The launch of a loyalty credit card scheme is the latest in a long-running overhaul of the company, aimed at updating the M&S brand and saving it from closure. Only four years ago, it refused to accept credit cards as payment for goods.
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