The Royal Mail has confirmed it is to close six main Post Offices across England and Wales and transfer their services to nearby WH Smith stores.
Customers will have six weeks to comment on the plans
Those Post Offices being closed are in Swansea, Slough, Shrewsbury, Ashton-Under-Lyne, Hammersmith, and one other location yet to be confirmed.
The Royal Mail said that if the move proved successful it could be extended to other UK Post Offices.
Unions have described the announcement as "blatant privatisation".
With its 500 main Post Offices making a £50m loss last year, the Royal Mail appears determined to offload a number of outlets.
The WH Smith stores concerned will take on the Post Office's full range of services.
The 120 affected Post Office staff will be offered voluntary redundancy or replacement work within the Royal Mail.
Meanwhile, customers in the six trial areas will be consulted over the plans during the next six weeks.
"This partnership between two trusted organisations is good news for customers and will enable us to both safeguard and enhance their Post Office services in the future," said Post Office programme director Byron Roberts
"WH Smith Retail's huge experience and knowledge will ensure that Post Office customers continue to receive a high standard of service."
A WH Smith spokeswoman said the company would like to take on Post Office work at more of its stores, but added that it was only possible in its largest outlets that had sufficient space.
She added that WH Smith aimed to have sufficient staffing levels on its Post Office counters to avoid any queuing issues.
The Communication Workers Union said the move was "disgraceful and scandalous".
WH Smith has 542 High Street stores across the UK and 127 outlets at railway stations and airports.