Post Office managers have voted to take strike action in a row over pensions.
Members of Unite backed walkouts by almost 2-1, and other forms of action by a bigger majority.
The union said around 3,500 staff are affected by plans to close the final salary pension scheme, claiming they will lose about 30% of their retirement income.
Unite officer Brian Scott said: "This strong vote in favour of industrial action should send a clear message to the Post Office that it has got it wrong over its decision to close the pension scheme.
"The Post Office's executive team and the board need to reflect on the closure of a scheme which is currently £100 million in surplus and the anger it is causing among a workforce which is also facing a large number of job losses.
"It is nothing short of pension 'robbery' and the anger has been compounded by a failing business plan underpinned by the continued franchising of Crown Post Offices and the outsourcing of vast swathes of the Post Office's supply chain business.
"Workers will not be made to pay for the failures of senior leaders in the Post Office with their pensions. Nor can the Government continue to bury its head in the sand in the belief that everything at the Post Office is OK when in fact it is a basket case.
"Unite will now be considering its next steps but believes that the clear mandate is a call for industrial action in the near future should the Post Office's leadership team fail to reconsider."
Kevin Gilliland, Post Office Network and Sales Director, said: "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.
"The business's financial position is improving but we remain loss-making. Based on the advice of our actuary, the fund's surplus, which is currently being used to help subsidise the cost of the defined benefit plan, will run out in 2017.
"Once this happens, the costs to the business of meeting existing commitments will significantly increase, and this is not sustainable. We therefore need to close the defined benefit plan to future accrual when the surplus runs out, because it is crucial that we safeguard the benefits that members have already built up.
"We've done what we can following consultation and having taken further advice from our actuary, to make changes to what we are proposing to try to lessen the impact on individuals. We are clear that our recommendation is the responsible thing to do both for members of the plan, and for the long-term financial health of the business. It is currently being considered by the trustee."
The Post Office said 97% of its 11,600 branches are not involved in the ballots for industrial action and said it would be working hard to minimise any disruption to customers in the around 300 Crown branches if industrial action was called.
A Business Department spokesman said: "As its annual accounts show, there has been a continuing improvement in Post Office's financial performance.
"Our investment has led to the most stable network for decades - with over 11,500 branches, 200,000 extra opening hours a week and more than 3,500 branches open on Sundays."