Communication Workers Union reps voted today to boycott private mail companies' post if regulator Ofcom doesn't stop them from "cherry picking" profitable services.
They called on Ofcom to protect the universal service, guaranteeing post costs the same across Britain.
The CWU warned of a "race to the bottom" as bosses squeeze postal workers' pay and conditions.
It called for minimum standards underpinned by the living wage.
Deputy general secretary Dave Ward said: "Competition is being allowed on the basis of low pay and unreliable job security in private companies.
"That will be a disaster for the sector if it continues. We will take action to protect jobs and services if the regulator doesn't fulfill its duty."
An Ofcom spokesman said: "Ofcom has not made proposals to reduce the universal service requirement from six days a week, nor do we have any legal powers to implement such a change."
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