Natwest Group PLC (LON:NWG) and Monzo are among a group of banks that have been warned about how they treat customers who switch.
UK competition regulator, the CMA said that the pair along with Virgin Money and Bank of Ireland had persistently failed to send departing customers with a statement of their final account.
This had affected their ability to borrow money or apply for a mortgage said the regulator.
"The Bank of Ireland, Monzo, Natwest Group, and Virgin Money should be in no doubt that the CMA stands ready to take further action if these failures are repeated," said Adam Land, a senior director at the CMA.
“The CMA’s work in the sector is making it easier for people to get a better deal from their bank. Banks must comply with all the rules – that includes providing a full transaction history promptly."
Monzo was cited in particular with the CMA saying that of the 150,000 customers affected, some 143,000 were from the challenger bank.
A Monzo spokesman told the Telegraph: "Since we identified the issue and reported it to the CMA in February 2021, we've fixed the problem and are telling affected customers that we can send them their transaction history if they'd like. “