Postal workers are being urged to refuse to deliver letters from anyone other than Royal Mail in the New Year, in a move which threatens to cause disruption.
But the Communication Workers Union (CWU) has claimed this arrangement costs Royal Mail money and has led to job losses, price rises and fewer services.
The CWU argues that competition on deliveries is "undermining" the same-price-goes-anywhere universal service, with companies other than Royal Mail not having to meet service and pay standards.
A ballot on a boycott will be held after Christmas.
If passed, 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."
His deputy, Dave Ward, added: "We are not prepared to stand by and watch the jobs of our members be ruined by unfair competition which could be avoided."
Mr Ward also told Sky News the union was opposed to privatisation of Royal Mail. He said: "No private owner is going to sustain the universal service that doesn't make money."
In response to the boycott threat a Royal Mail spokesman said: "All of the mail that we handle is important to us and needs to be delivered, as we always do, six days a week.
"It is vital to all of us at Royal Mail, to Royal Mail as a business and most importantly to our customers, that the post is delivered."
He added: "As we have previously said, we are concerned about the impact of unfettered direct delivery competition on Royal Mail.
"We will be responding to Ofcom's consultation on direct delivery in due course. Royal Mail supports competition on a level playing field and believes Ofcom should consider competition in the light of its primary duty of securing the six-day-a-week, one-price-goes-anywhere universal service."