The Post Office in The Boulevard, Crawley, is set to be moved into the WHSmith in County Mall.
When more details are announced, there will be a six week consultation, offering members of the public a chance to comment on the plans.
WHSmith already runs 135 Post Office branches inside its own stores. The recent deal is likely to see another 40 being relocated in 2019.
Meanwhile, WHSmith will be taking over the franchise of 33 Post Offices that are in their stores, but operated by the Post Office.
The Post Office says the move supports its ‘long-term sustainability’, helping them to keep a presence in the town centre’s main shopping area and may allow them to provide longer opening hours.
However, such moves have been controversial in many parts of the country.
Communication workers’ union CWU called the nationwide plan ‘nothing more than a privatisation by another name’ and said it puts 800 post office workers’ jobs at risk.
“The CWU is campaigning to keep post offices open and running in the midst of a controversial programme of closures,” said a statement from the union.
“A post office is not just a place for sending parcels, it brings the community feel to a town or village. A post office enables us to connect with millions of people and offers a variety of goods and services that without would see the community struggle.
“But due to closures, privatisation and neglect, this key part of British culture is under threat.”
Post Office network and sales director Roger Gale said: “We’re continuing to respond to unprecedented change on high streets and in consumer trends. By adapting to the needs of customers we’re making sure Post Offices will matter as much tomorrow as they do today, with services available when and where people want them, in convenient locations and open for longer hours, including Sundays.
“We’ve made significant changes in our network of 11,500 Post Offices over the past few years, modernising more than 7,500 branches and increasing opening hours for customers by more than 200,000 a week. We’ve also developed our services, including the introduction of everyday banking for customers of the UK’s high street banks.
“The vast majority of the Post Office’s network of 11,500 Post Office branches, large and small, are run on a franchise or agency basis with retailers as part of thriving businesses. It makes sense to further expand this successful, sustainable way of providing Post Office services to customers.”