Thousands of postal workers are to vote on strikes in a row over post office closures, job cuts and pensions, amid fears the service is nearing the "brink of extinction".
The Communication Workers Union has served notice that around 3,500 of its members in Crown offices, supply chain depots and administration sites across the UK will be balloted in the coming weeks on whether to launch a campaign of industrial action.
The union said the ballot is in protest at the company's plans for "wholesale privatisation" of the Crown network, job losses and the closure of the defined benefit pension scheme.
CWU general secretary Dave Ward said: "The Post Office as we know it is on the brink of extinction. It is clear that the board's plan is one of closure, redundancy and cost-cutting on an epic scale which will permanently undermine this great British institution.
"The Government's response on funding has been wanting and misleading - money has been spent on closing down offices and paying off staff instead of bringing new business into the network.
"We are left with no choice but to take strike action to defend the jobs and pensions of our members and the services they provide."
The CWU said there had been 2,000 job losses this year, with 16 Crown offices moved into another retailer and four closing. It added that the closure of the defined benefit pension scheme will leave thousands of staff worse off in retirement.
Its proposal will be the first time workers from the different parts of the business have been balloted together, with the result due on August 19.
The Post Office said in a statement: "The Post Office is committed to maintaining the vital services offered by its branches at the heart of communities across the UK, with 99% of the UK's population within three miles of a Post Office branch. However, this level of service can only be secured for future generations by ensuring the business's financial sustainability."
Kevin Gilliland, the company's network and sales director, added: "All of our proposals are taken forward with the utmost care for the people they affect and we're proud of our track record in supporting people through difficult changes.
"We will give serious consideration to any ideas that the CWU put forward to help us create the Post Office network our customers need for the future and urge them to work in partnership with us to secure the future of our services."