The Royal Mail is facing the threat of the first national strike since it was privatised after workers voted massively in favour of industrial action in a bitter dispute over pensions, pay and jobs.
Members of the Communication Workers Union (CWU) backed walkouts by 89% on a turnout of 73% of the 110,000 balloted.
It passed the threshold in the Government's controversial Trade Union Act, under which ballots need a 50% turnout for industrial action to go ahead.
The CWU believes it is a "watershed" moment for unions as well as the Royal Mail, which it has accused of following a "relentless" programme of cost-cutting to maximise short-term profits and shareholder returns.
The union accused the company of "unilaterally" closing its defined benefit, or final salary, pension scheme, with new entrants going into an "inferior" scheme which will leave them in "pensioner poverty".
The union is also in dispute over pay and issues such as delivery office closures.
Royal Mail said it was "very disappointed" by the ballot result.