The problems first surfaced this morning, with transactions being declined and incorrect balance entries displayed.

"One of our suppliers is experiencing problems which may cause declined payments when you try to use your card," the startup advised customers. "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."

The scale of the problems facing users became clearer as the morning wore on, with top-ups to the Monzo pre-paid card failing to load and P2P payments between Monzo users crashing.

In a later update, Monzo told customers: "We had to temporarily disable some app features: top-ups, Monzo to Monzo payments, and Monzo.me payments are currently unavailable. Card payments (including ATM withdrawals and online transactions) are still failing for a large proportion of customers. We're still working hard at a fix together with our supplier, and we will update you on a regular basis. Apologies to everyone."

It's not the second time Monzo has suffered at the hands of its suppliers. The bank promised to move card processing inhouse in March after a severe outage at current third party service provider GPS left customers high and dry.

As with that previous outage, other app-only challengers have similarly been caught up in today's black-out, including Revolut, Loot, Starling Bank, and Curve with GPS once again being fingered as the culprit.

The news comes a day after Monzo published annual results showing fast user growth and widening losses of £6.7 million.

Update With the issue resolved early afternoon, Suresh Vaghjiani, managing director of Global Processing Services (GPS), issued a statement: “A technical issue occurred on the GPS system on the morning of 6th July 2017 that impacted a proportion of transactions across several of our customers and many cardholders.

The immediate priority was to solve the issue and communicate directly and regularly with affected clients to support their ability to provide updates to their affected customers.

We provide our service to the leading fintechs in the industry who have selected our platform to allow them to innovate in a way that they were unable to do before. However, on this occasion we have fallen short of the high standards that we have prized ourselves on.

The issue is now resolved and we are analysing the root cause to prevent reoccurrence. We apologise for the frustration and inconvenience caused to our customers and their cardholders.”