More than a 100,000 Royal Mail workers have been asked to vote for or against the government's plans to sell off the "beloved British institution".
The Communication Workers Union (CWU) sent out ballot papers on Wednesday to 112,000 members who are postal workers asking them to "send a strong message to the government" and back the union's anti-privatisation campaign.
Dave Ward, deputy general secretary of the CWU, said: "We're asking postal workers to support the union or allow Royal Mail a free hand to determine their future – there is no fence to sit on.
"Support for the union's stance against privatisation will significantly strengthen the growing campaign against the sale of the business and protect the jobs of a loyal workforce as well as the services of a beloved British institution."
He added that a yes vote backing the CWU postion was the "only way to protect the interests of our members and the future of the UK postal service".
The results from the ballot, which also asks workers their opinion on pay and conditions and whether they would support a boycott of competitors' mail, are due on 19 June.
A yes vote could lead to the biggest disruption to the post service since industrial action six years ago as half of all letters in the daily postbag would not be delivered.
Royal Mail said it was "disappointed" that the union was going ahead with the ballot. "We are committed to seeking an agreement with the CWU on the way forward in Royal Mail that equips the business for the future and is fair to our employees," a spokesman said.
He warned employees that a boycotting delivery of rival firms' mail would "adversely impact the business, our reputation and that of our employees".
The vote comes just a day after Royal Mail boss Moya Greene pleaded with the CWU to stop the campaign against a selloff and "continue to drive our business forward as we seek to realise our collective objectives".
The CWU has repeatedly vowed to fight the sale, which it says will lead to a "worse deal for customers, staff and thousands of small businesses dependent on the Royal Mail".
Greene said privatisation was the only way the 497-year-old postal service could survive.
The business minister, Michael Fallon, who is in charge of the privatisation, said the government is "committed" to the sale, which could see the Royal Mail fall into foreign hands.
Vince Cable, the business secretary, said there was "no alternative" to privatisation and warned that Royal Mail still faces a "fundamental threat" from email, texts and social media.
Ward said Royal Mail's 60% increase in annual pre-tax profits to £324m, announced on Tuesday, was "more compelling evidence of why Royal Mail should be kept in the public sector".
"Privatisation isn't necessary and it would destabilise the workforce and the good progress being made. The support of the workforce is crucial to the success of the company," he said.
