Postal workers' leaders have decided to hold a national strike ballot if no agreement is made over jobs, pensions and other issues linked to the Government's controversial plans to privatise the Royal Mail.
Around 500 Communication Workers Union (CWU) representatives voted unanimously to press ahead with a strike ballot of 115,000 Royal Mail workers, no later than next month, if "satisfactory agreements" are not reached.
It will be the first ballot for national industrial action in Royal Mail since September 2009 and represents a major challenge to the Government's plans.
The union said it wanted to secure job protections in Royal Mail as well as maintain pay, pensions and conditions and stop any worsening of postal services.
Deputy general secretary Dave Ward said he thought it was "inevitable" that a ballot would be held.
Mr Ward revealed plans for a national day of action in support of postal services and against privatisation which could include a 24-hour strike.
He warned that selling Royal Mail would "damage" services and hit jobs, pay and conditions.
He accused the Government of "completely misleading" the public over privatisation, saying it was clear postal workers and the general public were against the plans.
"When they try to suggest that the postal industry has problems dealing with new technology and the digital age, they are talking nonsense," he told a special CWU conference in London.
"Workers have embraced modernisation, and helped increase profits by 60%, so why should we just hand it over to someone only interested in making money?
"We have a duty to defend the postal service - and we will."
Mr Ward said the coming months will also be a "pivotal moment" for the trade union movement as other unions are called on to support the CWU campaign.
Mr Ward said: "The current situation cannot go on. Postal workers are being squeezed in their workplaces, facing an uncertain future and changes to their pensions. There hasn't yet been a pay rise for staff this year despite healthy company profits of £403 million. But most importantly, we want protections for job security and terms and conditions and these are sadly lacking.
"CWU is committed to holding serious negotiations with Royal Mail to achieve settlement on these issues, but efforts to date do not bode well.
"The company only began to seriously negotiate with us following our consultative ballot in June, which showed 99% of postal workers back the union's position on pay, 96% are opposed to privatisation and 92% are willing to take part in a boycott of competitors' mail and to withdraw cooperation on workplace changes.
"We do not take the decisions to hold a strike ballot lightly. However, we will stop at nothing to ensure the future of our members' jobs - and of the services they deliver - are protected."
Mr Ward said he believed it was possible to stop the sale of Royal Mail, maintaining that the Government was worried about the union's response.
Privatisation was an "old policy" which was not the answer for the future.
"We are looking for a serious agreement which goes to the core of our fears about privatisation."
Mr Ward said the ballot timetable could be brought forward if the Royal Mail presses ahead with "scandalous" proposals on pensions.
"We cannot sit back. We want an agreement to shape the future of this company."
Royal Mail said it was “disappointed” at the decision, adding: “Talks are ongoing between Royal Mail and the CWU and we are committed to seeking an agreement. Since 2011, Royal Mail has been in talks with the CWU to reach a new agreement following the business transformation agreement in 2010.
“Royal Mail proposed a legally-binding three-year highly competitive agreement to deliver its agenda for growth and industrial stability that included an 8.6% increase in base pay over three years. It has been rejected by the CWU.
“We believe that a ballot on strike action is inappropriate. Disrupting the service Royal Mail provides to its customers is not helpful.
“Royal Mail operates in a very competitive market, especially in the parcels market. We recognise that customers have a choice and can move their business very quickly. We want to reach agreement with the CWU as soon as possible to give customers and employees continued stability.”