Post Office counter staff have voted by 9-1 to go on strike in a row over pay, jobs and office closures.
The Communication Workers Union (CWU) said turnout in the ballot of its 4,000 members at the so-called Crown offices was 66%, reflecting the strength of anger among staff.
The union has accused the company of not giving a pay rise to counter staff and refusing to give guarantees about the future of the Crown offices, the larger offices usually based in high streets.
Talks are due to be held in a bid to resolve the row, but the CWU warned that strikes could start by the end of the month if there is no progress.
CWU deputy general secretary Dave Ward said: "Post Office workers have sent a clear message to management in this ballot that they are not prepared to take double standards when it comes to pay. However, this ballot is about more than pay - it is the job security of our members and the future of the Crown office network which is also at stake.
"We want to see a strong network of Crown offices, but the Government and the company appear to be hiding the fact that they are planning a programme of further Post Office closures.
"Given the lack of support from the Government - only yesterday taking a key benefits contract away from the Post Office - we fear for the future of the network."
There were 2,365 votes in favour of strikes, with 172 against.
A Post Office spokesman said: "Post Office is disappointed that staff in our 373 directly managed (Crown) branches have voted in favour of strike action. Ninety seven per cent of Post Offices are run by subpostmasters and they will be unaffected by any industrial action.
"Postal deliveries across the UK are also unaffected by this ballot result."