A senior MP has written to the Royal Bank of Scotland and Lloyds Banking Group raising concerns about restrictions on some account holders using rivals' ATMs.
In August RBS revealed that around one million basic account holders would be prevented from using cash machines belonging to rival banks.
It argued that letting these customers use rival ATMs is unsustainable because it cannot recover the costs incurred each time a transaction takes place. Lloyds had already implemented a similar policy.
Andrew Tyrie, chairman of the Treasury Select Committee says MPs are concerned about the restrictions which could increase financial exclusion.
He has written to the chief executives of both banks asking them to clarify which services are being withdrawn, the rationale behind the plans, the associated cost savings, how many customers are affected and whether they remain committed to basic account provision.
"Your decision has major implications for access to universal banking services and appears to target vulnerable consumers who are most at risk of financial exclusion," say the letters.
RBS is 83% state-owned, Lloyds 43%.