The warning is issued on the day that the Communication Workers' Union sends out ballot papers to 140,000 members who are being urged to take industrial action to secure a reduction of the six-day working week to five days.
Both sides are convinced that the ballot is likely to produce a "yes" vote and the Labour Party is concerned that prolonged disruption could effect its electoral chances.
The Post Office memorandum, which was signed by Brian Thomson, personnel director of the Royal Mail, urges managers to undertake a massive "hearts and minds" campaign to persuade the workforce to vote against action.
The note says that senior management had pressed union representatives to resume negotiations which broke down in March.
Mr Thomson writes that the talks were not completed and that union concerns over the introduction of team-working could be addressed in further discussions.Reuse content