Poor access to current accounts and high bank charges hurt low income consumers, Citizens Advice has claimed.
Some accounts take 10 days to clear cheques
The charity said that banks were putting obstacles - such as tough identity checks - in the way of low income people trying to open accounts.
Nearly three million Britons do not have a current account; many have to rely on expensive high street cheque cashing stores to obtain cash.
The charity added that even those with accounts faced excessive penalty fees.
Citizens Advice accused banks of "merely paying lip service" to helping vulnerable consumers.
Under the banking code, UK banks are supposed to allow all consumers to open a basic current account.
These accounts give their holders access to a direct debit facility and bank ATMs but do not come with an overdraft facility.
However, despite government backing, one in 12 UK households still do not have a current account.
Citizens Advice said that banks were insisting on overly stringent identity checks for new customers and failing to provide account information in branches.
"Many of the most vulnerable are denied access to a bank account in the first place, or unfairly penalised when they open one," Teresa Perchard, Citizens Advice director of policy, said.
Ms Perchard added that some people with a basic current account had to wait up to 10 days for cheques to clear and were hit with harsh penalties if there was not enough money in the account to pay direct debits.
In response, the British Bankers' Association (BBA) told BBC News that banks took the providing accounts for low income consumers seriously.
"Since 2003 banks have opened 1.5m basic current accounts. Our research shows that the vast majority of consumers were satisfied with the process gone through to open the account and the functionality," Margaret Soden, the BBA's director of retail, said.
Citizens Advice, Which? and the National Consumer Council will appear in front of the parliamentary Treasury Select Committee on Tuesday as part of an investigation into whether banks are failing low income consumers.