Royal Mail will today file an injunction with the High Court if its main union fails to back down by 12pm from plans to hold a 48-hour walkout next week.
The Communication Workers Union (CWU) last week revealed plans for Royal Mail's first industrial action for eight years. The strike is set to start on 19 October.
Royal Mail said late on Friday it would mount a legal challenge to prevent the industrial action.
The company says that under "legally-binding dispute resolution procedures", five weeks of arbitration must take place before strikes can go ahead.
The CWU, which represents 110,000 postal workers, says such procedures have been fulfilled by a mediator being present at months of previous talks.
The union is striking over a raft of planned changes, including closing Royal Mail's "gold-plated" final salary pension scheme and changing working practices.