Royal Mail managers have voted to go on strike in a dispute over pay.
Unite said its members backed industrial action by almost 4-1 after rejecting a "paltry" 1.3% pay offer.
Unite officer Brian Scott said: "Our members have been disgusted by the abject failure of Royal Mail to take the issue of pay seriously, and this is reflected in the very strong vote for industrial action.
"We are already consulting our Royal Mail reps to decide what form the industrial action could take. We could be talking about an all-out strike which would cause severe disruption to deliveries to 27 million addresses but, in the first instance, we are likely to call upon our members to work to rule.
"Any action would have a detrimental impact on business and commerce."
Unite said around 4,900 managers were involved in the row.
A Royal Mail spokesman said: "Royal Mail is very disappointed that members of Unite have voted in favour of industrial action and action short of a strike over the 2015 pay offer for junior and middle managers. Any action or threat of action is damaging to our business.
"Royal Mail has proposed an average pay increase of 1.3% in base pay or a one-off payment equivalent to 1.3%, depending on a manager's grade.
"This offer follows several months of discussions with Unite. We believe the pay offer is fair and competitive and was ahead of inflation at the time the offer was made in September. It also compares well with other external pay offers in the UK.
"In the event of any action, we have robust contingency plans in place to ensure we continue to deliver a high-quality service."