All letters and parcels for delivery in the SE1 area will be processed in Croydon with effect from Monday 9 July, Royal Mail has announced.
The move, which affects mail for addresses in all SE postal districts from SE1 to SE28, is a result of the closure of the South London Mail Centre at Nine Elms.
Royal Mail has spent £10 million upgrading the Croydon Mail Centre in Beddington Farm Road to handle the extra work. Two more "intelligent letter sorting machines" have been installed.
"Customers should see very little change to their mail," says Royal Mail in an announcement published on its website.
"Deliveries and collections will continue to be performed from the usual delivery office."
For SE1 and SE16, the delivery office is located in Mandela Way.
Some local residents and businesses have recently received letters, sent from a Royal Mail office in Plymouth, announcing changes to delivery arrangements in Southwark and warning that mail may arrive earlier or later in the day than at present.
At Thursday night's South Bank Forum meeting Bermondsey & Old Southwark MP Simon Hughes announced that he will be meeting postal services minister Norman Lamb later this month to discuss residents' ongoing complaints about missed deliveries, delays and lost items in SE1.