The National Federation of SubPostmasters (NFSP) attacked a bid by the Post Office to ask its members to promote the sell-off.
The normally moderate federation said the Post Office was set to contact several thousand post offices in the coming days requesting they stock materials for the public on how to buy Royal Mail shares.
The federation urged its members to reject the call, pointing out that a privatised Royal Mail could be “catastrophic” for the post office network.
The NFSP has joined other groups in calling for a halt to the privatisation plans, warning that the government has failed to deliver a secure future for the UK’s post offices.
A third of all income earned by subpostmasters comes from sales of Royal Mail products and the NFSP predicts “mass” post office closures if that income is jeopardised.
NFSP General Secretary George Thomson said: “I am surprised and angered by POL’s request for subpostmasters to act against their own interests by encouraging the public to back the sale by buying shares.
“Subpostmasters are acutely aware that ministers are taking a reckless gamble with the future of our post offices by selling off Royal Mail.”