Postal workers could refuse to deliver post sent via anyone other than Royal Mail as part of a campaign to highlight a threat to the UK's universal service, union leaders have warned.
The Communication Workers Union (CWU) raised the threat of a boycott because of "unfair" competition, which it said had led to job losses, price rises and fewer services.
Competition on deliveries is "undermining" the same-price-goes-anywhere universal service, with companies other than Royal Mail not having to meet service standards or pay decent wages, said the union.
The CWU announced plans to hold a consultative ballot in the new year with the intention of boycotting competitors' mail.
This would mean that any mail sent via one of Royal Mail's competitors would not be delivered, the union warned.
Billy Hayes, general secretary of the CWU, said: "Today we're launching a major initiative to protect postal services in the face of mounting threats to jobs and services.
"Under unfair competition we've seen prices rise, services diminish, closures and job losses. Competition and privatisation are old-fashioned theories which have had their day. What's important is decent services and jobs and that's what we're standing up for."
Deputy general secretary Dave Ward said: "We are not prepared to stand by and watch the jobs of our members be ruined by unfair competition which could be avoided. Boycotting parts of the mail which are damaging services is a proportionate response to the threat posed by unfair competition. What we're seeing is private companies being able to do what they want with little concern for how it affects postal-services in the round."
The union said that under so-called downstream access arrangements, private companies could "cherry pick" profitable bulk mail contracts, taking revenue streams away from Royal Mail.
They sort and transport mail to a local Royal Mail office where they pay an access fee for Royal Mail postmen and women to deliver that mail - known as the "final mile".