Smile Bank customers have been unable to access their online banking accounts for five days in a row.
The online-only bank, owned by Co-Operative Bank, said it was working urgently to resolve the problem.
But the latest service outage proved to be the “last straw” for longtime customer Emma Hopkinson-Spark.
After 16 years, she left Smile Bank on Friday after she was unable to view a crucial re-mortgage payment.
“I’m really disappointed in Smile Bank,” Ms Hopkinson-Spark, chief of staff at a tech consultancy business in Weston-super-Mare, told the BBC. “The outages this week were the tipping point.”
Ms Hopkinson-Spark said she had not been able to access her online banking account for several days. Although she can still withdraw money from cash machines and pay for items using her debit card, she cannot send payments or view her payments.
“Yesterday my husband rang me in a flap saying he