Postal workers are to be balloted for national strikes over issues linked to the government's controversial plans to privatise Royal Mail.

The Communication Workers Union (CWU) said 125,000 of its members would vote in the coming weeks over pay, jobs, pensions and the impact of any sell-off, with the result due in early October. The first strike could be held on 10 October if there is a yes vote.

The move is a major challenge to the planned privatisation, which ministers are expected to press ahead with in the coming months. The union warned a strike was inevitable unless a deal was agreed on a number of issues.

The CWU is in dispute over a pay claim, further changes to workers' pension schemes, the impact of possible privatisation on job security and terms and conditions, and the company's future strategy.

All Royal Mail and Parcelforce workers in the CWU will be balloted from 20 September, with the result due on 3 October. If there is a yes vote, the union would have to give seven days' notice for strike action.

Dave Ward, the CWU deputy general secretary, said: "We are dealing with a company that is preparing for privatisation with relish. While the union continues to fight privatisation we are also dealing with the potential realities for workers if there is a change of ownership.

"We are looking to reach a groundbreaking agreement on terms and conditions that sets unprecedented legally binding protection for workers in the event of a sale, and regardless of who owns the company.

"Postal workers know franchising, breakup and sale of mail centres, distribution hubs and Parcelforce, along with the introduction of a new workforce on lower terms and conditions, are real threats in a race to the bottom with mail competitors for any new company.

"We want Royal Mail and the government to put protections in place that are both meaningful and lasting.

"Royal Mail continues to prepare for privatisation with relentless rounds of budget cuts in offices across the UK. There is no understanding that the pace of change can really only be led by how hard people can work, and CWU members are being driven to absorb absences, carry increasing amounts of mail and work harder than is possible in many cases.

"We have reached breaking point, particularly in delivery offices, and the culture has to change."

It will be the first national postal ballot since a pay and conditions dispute in 2009.

The union said it had rejected a below-inflation pay offer linked to accepting major changes to working conditions and pensions.

A number of local disputes have broken out, with the union warning that budget cuts are stretching workers "to the limit".

The CWU added that workers were also concerned about pensions, with assets being used to reduce Royal Mail's contributions.

CWU members voted in a recent consultative ballot by 96% against privatisation, with a similar number saying they would be prepared to boycott delivering competitors' mail.

Royal Mail managers in the Unite union have also voted strongly against the planned sell-off.

CWU members working in Post Offices will not be involved in the ballot, although they have been taking strike action in a separate row over closures, jobs and pay.

A Department for Business spokesman said: "Industrial action is not necessary. It is disappointing that the CWU leadership has decided to ballot for strike action.

"They are standing between their members and a generous pay offer of 8.6% over three years, which is more than teachers, nurses and our armed forces, who have had pay increases capped at 1%.

"Royal Mail management are continuing to talk to CWU and we encourage both sides to resolve this dispute.

"Action taken by the CWU will not alter the government's decision to sell shares in Royal Mail in this financial year.

"Parliament decided over two years ago that selling shares in Royal Mail was the right thing to do to secure Royal Mail's future and protect the six-day-a-week universal postal service.

"A successful, financially sustainable Royal Mail with access to private capital is in the best interests of the workforce and all users of the universal service."

A Royal Mail spokesman said: "Royal Mail is very disappointed that the CWU has issued a timeline for a national ballot for industrial action, if an agreement is not reached in ongoing talks.

"Discussions over a new three-year agreement between Royal Mail and the CWU are continuing. We are committed to reaching an agreement with the CWU as soon as possible to give our customers and employees continued stability. We believe that focusing on the possibility of industrial action is inappropriate.

"A ballot for strike action does not mean there will be a strike; currently it is business as usual for Royal Mail. Any industrial action, or the possibility of disruption, is damaging to our customers, and our business, especially in the runup to our busiest time, Christmas.

"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 quickly.

"Royal Mail will do all that we can to protect our business and our customers' mail. We will work hard to minimise the impact of any industrial action on our customers."