Households risk being hit by postal chaos with the threat of a boycott by Royal Mail staff.
Union leaders say up to 26 million items could go undelivered every day if a ban on handling letters from rival operators goes ahead.
Some Royal Mail workers are furious that firms such as TNT and UK Mail can “cherry pick” lucrative contracts with big firms to collect and sort their post in bulk.
These companies then hand it on to Royal Mail to deliver to homes across the country.
The Communication Workers Union fears that Royal Mail will suffer serious long-term damage if rivals go one step further and set up their own door-to-door deliveries.