The main post office which serves Motherwell could be on the move as part of a plan to sell off and franchise 37 branches nationwide.
The Crown office branch in Brandon Parade is one of five in Scotland facing an uncertain future.
Officials said the plans would underpin their commitment to “give communities in every part of the country access to essential services”.
But the Communication Workers’ Union (CWU) said it could see the loss of 300 staff and cut 127 financial specialist roles.
The union’s general secretary David Ward said it was “further evidence that the Post Office is in crisis”.
In recent years there has been a move to close sub-post offices and move the services within retail premises. This often gives customers seven days a week service and extended opening hours.
In a statement the Post Office said: “Our network of more than 11,600 branches is easily the largest in the UK, with 17 million customer visits a week.
“The vast majority of these branches are run with partners, and in the locations announced we believe this will also be a more sustainable approach for the long term.
“With consumer habits changing, and the high cost of maintaining premises in prime locations, franchising helps us to keep services where our customers want and need them.”
The Post Office will now advertise for potential partners for the next 28 days. If interest is registered this will be considered before any proposals are shared with staff.
Six weeks of public consultation will then follow before any changes are made, according to a Post Office spokeswoman.
The CWU said 75,000 postcards were recently sent by the public to the government calling for an end to the closure and franchise programme.
A union spokesperson said: “The announced franchising of five Crown Post Offices in Scotland represents a major loss to the local communities. Like many Crown Offices throughout the United Kingdom, they provide a vital public service for the local community and it is part of the fabric of our society.
“The loss of this service would be detrimental to Scotland in particular.”