Crown Post Office workers were to begin balloting for strikes on Christmas Eve from Friday of this week.
Bosses refused to give workers their usual Christmas Eve early finish.
Ballot results are set to be announced on Monday 10 December, leaving the union just enough time to give notice for a strike on Christmas Eve.
The ballot comes as Crown Post Office workers were also set to take part in a day of protests against privatisation on Saturday of this week.
Post Office bosses want to sell 74 flagship Post Offices to WHSmith—affecting as many as 800 jobs.
Protests against outsourcing
Members of the CWU union working for the BT telecoms company were taking part in a week of protests across Britain this week.
They are fighting plans by BT bosses to outsource hundreds of facilities services workers.
Bosses want to transfer some 1,700 workers, who perform jobs such as catering and maintenance at BT offices, to private companies CBRE and ISS.
Workers fear this could lead to attacks on their terms and conditions—and more outsourcing for other BT workers.
Strikes are the best way to stop outsourcing.
CWU members protested at BT sites in Doncaster and central London last week.
More protests were set to take place in London, Leicester and Sevenoaks this week.