In a notification sent to customers, the startup said there have been issues since just before 10 am on Sunday morning due to one of its suppliers running "an unexpected large migration" with no estimate of when services will return.
The issue is also affecting Revolut, another fintech startup which issues cards to customers.
Both firms are advising customers to use another card and apologised for any inconvenience.
UPDATE: Due to our card processor server issue we advise you to keep another card with you for the rest of the day. We’ll keep you posted.— Revolut (@RevolutApp) March 5, 2017
"Your money is safe and this is only a temporary issue," Monzo assured customers.
"We will be conducting a full review of how and why this happened and what can be done, both us and the supplier, to ensure this does not happen again."
Many users on Twitter aired their annoyance with the disruption. However, one user even gave Monzo a thumbs up for its communication.
"Thanks for the info, you're the first bank I've used that give a push notification when stuff isn't working. Thumbs up".
Update: A third startup in the fintech space, Loot, has also been affected.
⚠️We’re experiencing an issue relating to payments & transactions. We're working to have this fixed within the next few hours! ⚠️— Loot (@LootApp) March 5, 2017
Update: A fourth firm, Curve, has also been affected
Update: In its most recent update to customers, Monzo said card payments had started working again but topups, payments to other users and monzo.me were still not functioning.
Update: Full service has been returned to Monzo early on Monday morning, with the majority of services also returned to other firms affected.