American Express has moved out of offices in Preston Road, Brighton, relocating 300 employees to Burgess Hill and other sites in the city.
About 250 financial and human resources workers from Prestamex House have been moved to Sussex House in Burgess Hill.
The other 50 employees have been relocated across Brighton to either Napier House in Trafalgar Place or the company's European headquarters in Amex House, Edward Street.
Earlier this month, it was revealed that an undisclosed number of workers from Napier House had already been moved to Amex House as part of a company reorganisation.
A spokeswoman for Amex said: "The company has moved out of Prestamex House entirely during the last few weeks.
"We want to emphasise that no jobs have been lost and that staff have been relocated to our other sites, where spaces have been freed up."
The international finance firm moved 150 workers to Sussex House last year, with the remaining staff being relocated during the last month.
Eight-storey Prestamex House is now being sub-let.
Amex, which employs more than 3,000 people in Sussex, sparked concern this month by setting up a call centre in India.
The company has refused to reveal how long the scheme will run or what will happen if it proves a success.
Although Amex does not recognise trade unions, dozens of its employees are GMB members and have been in daily contact with the Brighton branch, concerned about their jobs.
Gary Smith, of the Brighton branch of the GMB, said: "This branch closure is certainly a very interesting development."
Amex set up its European headquarters at Amex House in 1977. From there it operates card and travellers' cheque transactions from across Europe, the Middle East and Africa.
The Finance Central Centre at Sussex House provides jobs for about 1,000 people and deals with finance, taxation and treasury for the credit card company.
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