Up to 61 more Post Office branches will move into WHSmith stores over the next year.
Around 300 so-called Crown offices currently remain on the high street after 107 were transferred to the retailer over the past decade.
Announcing a new 10-year agreement with WHSmith, 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 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.
Locations are yet to be confirmed and Mr Gale said that staff will be informed first about plans for their own branches when these are finalised.