Customers of several British digital finance companies were unable to make payments or access their money on Friday after the collapse of German payments firm Wirecard caused disruption across borders.
Wirecard's implosion on Thursday, owing creditors almost $4 billion, led Britain's Financial Conduct Authority to impose restrictions on the company's UK unit.
That in turn forced firms that had been relying on Wirecard services to temporarily suspend their own, leading scores of customers to complain on social media about losing access to vital services - and money.
"Is my money safe? When can I get it back? Won't let me withdraw or pay a supplier. What the hell is going on???" tweeted a customer of business account provider ANNA.
"It's awful I need my money as due to reopen my business next week," said another ANNA customer.
ANNA said in a statement that it had had to temporarily suspend customer cards and accounts and was working to restore services as soon as possible.
Card provider Curve said it had seen a temporary disruption to services and advised customers to use alternative payment methods, while account provider Pockit told its customers accounts would be inaccessible for a short period and it was working with the FCA to find a solution.
The FCA said so-called safeguarding rules protect and return customer money if a firm were to fail.
"Wirecard is required under the Electronic Money Regulations to maintain appropriate measures to safeguard customers' money," the UK financial watchdog said.
Sarah Kocianski, head of research at fintech consultancy 11:FS, said the knock-on effects of Wirecard's collapse posed a big test for digital firms that often rely on backend services provided by bigger players.
"This could rebound not only on the companies in question, but also on the wider fintech industry as consumers question the newer providers they have recently adopted in large numbers."
(Reporting by Iain Withers and Anna Irrera, Additional reporting by Huw Jones; editing by Philippa Fletcher)
