Up to 61 more Post Office branches will move into WHSmith stores over the next year, it has been announced.
The so-called Crown offices are usually sited on high streets.
Around 300 Crown offices remain after 107 were transferred to the retailer over the past decade.
Post Office chief executive Paula Vennells said: "We know that people place huge importance on the presence of a Post Office branch on their high street.
"Our collaboration with WHSmith will help to secure this, as well as bring new investment into the network."
The Post Office announced a new 10-year agreement with WHSmith to relocate more branches into the retailer's stores, which it said would keep services on high streets.
Just over half of the branches changing over are expected to be concession agreements run directly by the Post Office.
The plans will include some of the 39 directly run Crown branches that the Post Office announced proposals to franchise in January, but locations are yet to be confirmed.
Roger Gale, general manager of the Post Office's Crown branch network, said WHSmith franchises had improved customer service and brought benefits, with most open Saturday afternoons and Sundays.
"The vast majority of the Post Office's network of 11,500 Post Office branches is already run on a franchise or agency basis," he said.
"Crown branches are still run by directly by the Post Office but factors such as high property costs and changing customer habits mean we have to consider new ways to provide our services in some locations.
"Today's agreement means we can maintain and improve services from convenient high street locations in a commercially sustainable way. It's great to be working with a partner that we know shares our values and is as committed to thriving Post Office branches."
Mr Gale added that staff affected by the new agreement will be informed first about plans for their own branches when these are finalised.