© REUTERS/Michael DalderFILE PHOTO: The headquarters of Wirecard AG, an independent provider of outsourcing and white label solutions for electronic payment transactions is seen in Aschheim near Munich, Germany April 25, 2019.
The UK licence of Wirecard Card Solutions has been frozen by the regulator after its German parent company filed for insolvency.
It means people are temporarily unable to access cash held with financial apps in the UK using Wirecard technology.
Some have spoken of their frustration, but their money should be safe.
Comment: That doesn't sound like a guarantee.
Wirecard Card Solutions serves prepaid cards, such as the U Account, which marketed itself as an alternative to a bank which helped people to budget and avoid hefty overdraft fees.
"This is a massive issue for many customers who rely on the money placed in their U account for everyday essentials including the ability to pay rent and buy food," one customer told the BBC.
'Money is safe'
The regulator, the Financial Conduct Authority (FCA) said it had ordered Newcastle-based Wirecard Card Solutions "to cease all regulated activities in order to further protect customer money".
Financial technology firms Pockit; Anna Money, a business account; and Curve, which aims to consolidate people's finances in one app, told customers they would be blocked from their accounts after the FCA's decision.
They said that customers' money was still safe, but would not be accessible for a time.
"This action is not related to Curve - but Curve currently depends on Wirecard for operation of the Curve card," Curve told customers on Friday. "We are recommending that for the period of this short suspension Curve customers carry an alternative payment method," it added.
Comment: How can they pay with an alternative method if their actual money is being withheld?
"We are already well on the way to migrating away from Wirecard but have not fully completed this process."
Anna said: "We expect the suspension to be lifted - the inability to use your account and card is temporary, and we are working to restore it as soon as possible."
Pockit said it was working with the FCA to get the issue solved as quickly as possible.
On its website, Wirecard Card Solutions said: "Wirecard Card Solutions Limited (WDCS) has temporarily suspended its electronic money issuing, card issuing and acquiring business with immediate effect and until further notice.
"WDCS is working hard to have the steps in place which will enable the suspension to be lifted so business can resume as usual. We will provide further updates on our website as soon as we can."
Funds are not protected, as they are in banks, by the Financial Services Compensation Scheme, but the firm said customers' money was held safely in segregated accounts.
The FCA has told people affected to contact their card provider.
The action comes after the German parent firm Wirecard last week disclosed a €1.9bn (£1.7bn) hole in its accounts, and subsequently filed for insolvency.
Former boss Markus Braun has been arrested and accused of inflating Wirecard's finances to make them appear healthier to investors and customers.
The Munich based firm employs almost 6,000 staff in 26 countries. The firm's creditors stand to lose billions of euros from the scandal.
Comment: See also: Billions of euros confirmed missing from Germany's Wirecard