The UK bank TSB has informed its approximately 5,000-staff workforce that they will be required to work in an office three days a week from April 2027.
The bank's decision has sparked a dispute with the TBU union, which is preparing to take cases to the Employment Tribunal in response.
The bank has launched talks with staff and management to establish exceptions for individuals with personal and health issues requiring a more flexible approach.
Under government guidance on flexible working, employees cannot complain to a tribunal solely because a request has been refused, but can do so where an employer has not handled a request in a reasonable manner.
Complaints must be made within three months of the employer's decision, and the maximum award is eight weeks' pay.
The row is the latest example of a clash within a business about plans to get staff back into the office more regularly following the Covid-19 crisis.
Santander, the parent company of TSB, has already tightened its hybrid working policy, telling 10,000 UK office-based employees to work the equivalent of three days a week at its sites, up from two.
TSB is undergoing an integration process with Santander, which includes making 130 redundancies.
The integration process is being led by Mahesh Aditya, chief risk officer of Banco Santander, who took charge of Santander UK at the beginning of March.
TSB has become mired in controversy over technical difficulties from past operational overhauls, including a recent IT platform transition that caused chaos for millions of customers and months-long disruptions to its services.
The bank was formerly known as Trustee Savings Bank and was first established by the Rev Henry Duncan in Dumfriesshire in 1810.
TSB was maintained as a brand when the lender merged with Lloyds Bank in 1995, and was later acquired by Sabadell, a Spanish bank.
Sabadell acquired TSB at book value for £1.7 billion in 2015, before agreeing the sale to Santander.
Santander first entered the British market in 2004 with the purchase of Abbey National for £9 billion and expanded its footprint during the 2008 financial crisis with several acquisitions.
