Update 11.15am: Since we published this story the technical issue now appears to have been resolved and cards are working normally.
Prepaid card providers Monzo, Revolut and Curve have warned customers that their payments may be declined due to a fault.
In a statement on its website this morning, Monzo said: "We are currently experiencing issues with card payments and your transactions may get declined. We are currently investigating and will provide an update as soon as possible.
"One of our suppliers is experiencing problems which may cause declined payments when you try to use your card. The app may show the payment as successful even though it was not, and the amount may appear to have been deducted from your balance.
"This is a display error – your balance 'behind the scenes' has not been debited. We'll post more information as we hear more. Apologies for the inconvenience."
The latest updates can be viewed here.
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Revolut, Curve, Starling and Loot also affected
As in March 2017, when a similar issue hit Monzo, other prepaid card providers are also affected.
Revolut, Curve, Loot and mobile-only bank Starling have all suffered problems this morning, as well as Monzo.