Monzo, the cult challenger bank, has apologised to customers over a spate of outages, saying the issues "may continue for at least the next few days".
In an email to customers today the company said its technical issues, which last "anywhere from a couple of minutes to over an hour", were due to its card processor experiencing technical problems it "hasn't yet been able to solve".
The move followed a similar email at the beginning of last month, when it advised cardholders to use another card after an issue with a supplier.
Monzo warned customers to carry a backup card from another bank with them.
"Unfortunately, we don’t know how long it will take our card processor to fix the issues, but we will always keep our status page up to date," it said.
"People across the UK have come to rely on their Monzo cards and if at times you can’t use your card, you lose trust in us. These outages are unacceptable. I want to personally recognise that and apologise on behalf of the whole company.
"There is a glimmer of hope, though. Our in-house Mastercard card processor is now live and current account debit cards have been working well for the last couple of months, unaffected by these issues. We believe it will be a more stable base on which to run the bank."
In July Monzo published its annual report, saying it hoped to be able to have between 500,000 and 800,000 current account customers by the end of the year.
The service currently only offers a pre-paid card, but has said it hopes to begin offering a current account by the end of the summer.