More than 115,000 Royal Mail workers have voted to go on strike in a dispute over pay.
Members of the Communication Workers Union (CWU) backed the action on Tuesday after ballot papers were sent out three weeks ago.
Any strike dates are yet to be decided but the CWU said if a walk out goes ahead, it could amount to the biggest ever action taken by its members.
Royal Mail has previously said there are “no grounds” for strike action.
The CWU is the latest of several unions to ballot for strikes in recent weeks as the cost of living soars.
The union said postal workers “will not budge” until they receive a “dignified, proper pay rise”.
Some 97.6% of members from a 77% turnout voted in favour of striking.
The union has demanded Royal Mail group enters into negotiations to secure a “straight, no-strings pay increase for workers”.
Royal Mail previously said it offered workers a “deal worth up to 5.5% for CWU grade colleagues, the biggest increase we have offered for many years”.
Dave Ward, CWU general secretary said Royal Mail’s chief executive and board “should seriously consider their futures” as the ballot was “also a vote of no confidence”.
“While bosses rake in £758m in profit and shareholders take £400m, workers are expected to take a serious real-terms pay cut,” he added.
“Postal workers won’t accept their living standards being hammered by bosses who are typical of business leaders today – overpaid, underqualified, out of their depth.”