Royal Mail is facing its first national postal strike in a decade after staff voted overwhelmingly for action.
The dispute between workers and the firm is over job security and terms and conditions of employment.
More than 97% of votes by members of the Communication Workers Union (CWU) backed a strike. Turnout was 76%.
Strike dates have yet to be announced, but the union could target the annual Black Friday retail sales event in late November and the Christmas post.
The CWU says an agreement reached with management last year to raise pay and reform pensions is not being honoured.
About 110,000 members of the union were balloted in the dispute.
Royal Mail says it has 51% by volume of the UK parcel market.
Terry Pullinger, deputy general secretary of the CWU, accused Royal Mail of breaking the "progressive" agreement that it reached with the union a year ago.
He added: "Our members take honour seriously and have voted to fight for that agreement against those who now seek to break up the great British postal service in the interest of fast-track profit and greed."
The CWU's general secretary, Dave Ward, urged Royal Mail to enter "serious negotiations" with the union.
Royal Mail said it was "very disappointed" that the CWU had chosen to ballot for industrial action and said it was still "in mediation" with the CWU.
"We want to reach agreement. There are no grounds for industrial action," the firm said.
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 Back.
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.