The Treasury Committee has demanded answers from fintech firm Cashplus after outages left customers unable to make payments or withdraw cash.
Customers had complained of being locked out of their accounts since the beginning of the week and unable to make or receive payments.
Nicky Morgan, chair of the Treasury Committee, has now written to Cashplus chief executive, Richard Wagner, asking for an explanation and what steps will be taken to compensate customers.
She said the fintech firm marketed itself to people with poor credit histories and small businesses, to whom a loss of banking services would carry “particularly severe consequences”.
Morgan said: “While Cashplus may not be regulated as a bank, it claims to offer 'the UK's most seamless banking services experience'.
“Its customers rightly expect continuous access to their accounts, and it should be held to the same standards of operational resilience as a retail bank.”
The fintech startup first apologised on Tuesday for the outages affecting its third-party card processor and said its team was “working tirelessly” to fix the problem.
The company reiterated its apology earlier today and said its services had returned to working as normal.
The Treasury Committee has also quizzed PayPoint, Visa and TSB over service failures in recent months.
Morgan asked for a response by 5pm on 28 September.