Thousands of Royal Bank of Scotland customers have been barred from using cash machines operated by the firm's high street rivals.
Changes brought in by RBS last month to its "basic" account have resulted in customers only being allowed to withdraw money from ATMs operated by the company.
Those signing up to the "basic" account pay no fees and are already unable to hold a chequebook or have an overdraft.
RBS would not confirm how many people are affected by the move, but it could be as many as 8,000.
A spokesman for RBS said: "We are fully committed to offering a free basic account for people who may otherwise struggle to access banking services.
"These account holders will continue to have free access to one of the largest cash machine networks in the UK.
"Our basic account holders will still enjoy free access to one of the largest ATM networks - over 8,000 - in the UK, through RBS, Natwest and Tesco.
"They also can make withdrawals at any Post Office counter."
Marie Burton, financial inclusion expert at Consumer Focus, said: "This is a kick in the teeth for many of RBS's most vulnerable customers.
"We would call on RBS to reconsider this move which will exclude its basic bank account holders from using the majority of the UK's free cash machine network."
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