Royal Mail staff have voted overwhelmingly in favour of a national strike in a dispute over job security and terms and conditions of employment.
More than 97% of members of the Communication Workers Union backed a strike. Turnout was 76%.
The CWU says an agreement reached with management last year to raise pay and reform pensions is not being honoured.
It means Royal Mail is now facing its first national postal strike in a decade.
Strike dates have yet to be announced, but the union may choose to target Black Friday and the Christmas post.
Royal Mail says it has 51% by volume of the UK parcel market.
Strikes at the privatised postal service were averted last year after Royal Mail agreed to raise pay, reform pensions and reduce weekly working hours from 39 to 35 by 2022, subject to productivity improvements.
However, the CWU has claimed that the deal is “under threat” under recently appointed chief executive Rico Black.
Royal Mail has said it is abiding by the agreement and has awarded two pay rises since last year.
It also said it had cut the working week by an hour – although discussions with the CWU about further cuts had stalled.