Traders say they will fight a decision to move a busy city centre post office into a corner of WH Smith.
Businesses in the commercial heart of Brighton and Hove said they would suffer as a result of the costcutting measure by Post Offices Ltd.
The company claims the popular Ship Street branch in Brighton must close as part of a drive to stem £70 million losses.
It will be transferred to a portion of the WH Smith store in Churchill Square before autumn next year.
Traders in the Lanes said their web businesses, as well as shoppers, residents and tourists, would all lose out if the move goes ahead.
Stuart Wilkie, chairman of The Lanes Business Network, said: "Everyone agrees it's an extremely busy post office so why do they need to relocate it?
"Many of us have mail order and ecommerce businesses that rely on the post office to get our products out.
"There's no way we can all walk up to Churchill Square and then wait around for half an hour stuffed in the back of a busy store.
"We will appeal to them to reconsider and if not we pledge to fight."
Post Offices Ltd is moving 76 of its crown post offices across the country to WH Smith stores.
The Ship Street building, which the post office moved into less than three years ago, will be sold.
The staff of 22 employed there will either be able to apply for a position at WH Smith, move to another job within Royal Mail or take voluntary redundancy.
Customers will be asked which services they use the most and what they would like to see changed when the post office moves.
Mr Wilkie said a meeting of The Lanes Business Network last week decided "whole-heartedly and 100 per cent" to fight the decision.
Soozie Campbell, city centre manager, said: "Lots of businesses and offices in that area use it."
Traders will be joined in the battle to save the service by councillors and politicians.
Brighton Pavilion MP David Lepper has written to Alan Cook, managing director of Post Offices Ltd, concerned over possible job losses and the effect on North Laine and The Lanes.
He said: "They seem to be more concerned in the business of property dealing rather than the services needed by Brighton and Hove."
Gill Mitchell, chairwoman of Brighton and Hove City Council's environment committee, said she hoped if enough businesses and residents protested against the plans they may be able to secure a "stay of execution".
A spokesman for the post office said: "Post Offices Ltd loses £70 million a year through its crown network.
Whether a particular branch loses money or not is confidential but clearly these losses are unsustainable.
It's a commercial decision.
"The public consultation held before the move will be about levels of service but the decision to move has been made in principle."
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