The Royal Mail is to seek an injunction to try to stop a planned 48-hour strike by postal workers in a dispute over pensions, pay and jobs.
Members of the Communication Workers Union are set to walk out from October 19 after voting massively in favour of industrial action in a long-running row.
Royal Mail wrote to the CWU, invoking a "legally-binding external mediation process", saying any industrial action would be unlawful.
The union was given a deadline of noon on Monday to withdrawn the strike threat.
The Royal Mail said in a statement: "The company requested that CWU withdraw its notification of industrial action by 12 noon today and commit to following the dispute resolution procedures.
"CWU has declined to withdraw its notification. As a result, Royal Mail will today lodge an application with the High Court for an injunction to prevent industrial action so that the contractual external mediation process can be followed.
"A date for a hearing will be arranged with the High Court.
"As previously stated, we believe any strike action before the dispute resolution procedures have been followed would be unlawful strike action."