Virgin Money's St Albans branch is among those set to close by the end of year, the lender has announced.
"Changing customer demands" were blamed for its decision to close 38 sites across the UK, with up to 260 staff at risk of losing their jobs, according to Unite.
The branches are expected to close by the end of the year, with the St Peter's Street site among them.
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"Each store was assessed on an individual basis, with careful consideration of the impact on the local area, as well as the needs of vulnerable customers and the accessibility of alternative services such as free-to-use ATMs and Post Offices," said a spokeperson for Virgin Money.
"Each store closing is less than half a mile from the nearest Post Office, which customers can use to carry out day‐to-day transactions, including cash deposits and withdrawals, cheque deposits and balance enquiries, as well as coin exchange."
Unite national officer, Caren Evans, added: "The staff at Virgin Money UK are understandably devastated.
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"Unite has urged the bank to rethink these catastrophic plans to withdraw banking services from communities who very much depend on the skilled and experienced banking staff.
"This decision to pull out of these locations will hurt some of the most vulnerable, disabled and digitally excluded customers."
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