Barclays has announced 200 employees in Leeds are to lose their jobs.
The bank said it would also be closing its office in the Beeston area of the city by the end of the year following the landlord's decision to sell it.
Another 270 workers will be relocated to offices in Manchester and Sunderland. A further 100 will relocate within Leeds, the firm said.
The Unite union said the decision to close the site would be "devastating for workers and the local community".
Barclays has not confirmed its plans for the remainder of about 700 permanent staff at the site, which deals with mortgage cases, lending operations and customer complaints.
A spokesperson for the bank said: "We will do everything we can to support colleagues impacted by the changes announced today, working closely with Unite."
Dominic Hook, national officer for the Unite union, said: "The sheer scale of the job cuts announced today by Barclays is alarming.
"Unite has argued for some time that the bank has a social responsibility not to walk away from its committed workforce in Leeds.
"Today is indeed a dark day for [those] who have worked relentlessly to deliver the highest customer service for Barclays customers."
The union also claimed hundreds of other workers would be affected by plans to relocate them to other sites.
"To add to the misery today within Barclays, the news of a further 345 staff cuts in over 20 locations is deeply troubling," Mr Hook said.
He claimed 157 of the jobs were in Coventry.
Barclays said only about 50 jobs would go, with the majority of workers relocated to key sites in Glasgow, Greater Manchester and Northampton.
"This will enable us to innovate at pace for the customers and clients we serve," it added.