Royal Mail today confirmed it will file an application with the High Court for an injunction to prevent a proposed walkout by postal workers next week.
The 501-year-old firm said today's 12pm deadline had passed with the Communication Workers Union (CWU) refusing to remove its notification for industrial action.
A 48-hour walkout is due to start on 19 October.
Royal Mail said: "We believe any strike action before the dispute resolution procedures have been followed would be unlawful strike action."
A date to hear the application is yet to be determined.
The CWU, which represents around 110,000 postal workers, and Royal Mail signed up to an "Agenda for Growth" side agreement in 2014. Within it, concessions were made by Royal Mail such that CWU committed to five weeks arbitration before going on strike.
The union believes mediators were present at previous talks prior to balloting members for action. Such representatives, it argues, fulfil the requirements laid in the side agreement.
Royal Mail today stressed 94.5 per cent of CWU members voted in favour of the agreement.