The Royal Mail will take legal action if the postal union does not call off a planned 48-hour strike by Monday.
The Communication Workers Union (CWU) is planning a walkout of 110,000 of its members from October 19 in a bitter dispute over pensions, pay and jobs.
Union members voted massively in favour of industrial action in a ballot.
Royal Mail said it has written to the CWU invoking a "legally-binding" external mediation process.
A statement said: "Royal Mail requested that CWU withdraw its notification of industrial action and commit to following the dispute resolution procedures.
"If CWU does not withdraw its notice of strike action by 12 noon Monday 9 October, Royal Mail will lodge an application with the High Court for an injunction to prevent industrial action."
The Royal Mail said it believed any strike action before the dispute resolution procedures have been followed would be unlawful.
Union members had agreed to back a so-called Agenda for Growth agreement in 2014, which included the dispute resolution procedures, said the company.
"External mediation as set out in the Agenda for Growth agreement has not yet taken place," said the statement.
"A clear mechanism for instituting these procedures is set out in writing in the Agenda for Growth.
"It includes: a) Formal notice of a referral to external mediation from one party to the other; b) The appointment of an external mediator through the mutually agreed process and; c) A comprehensive mediation process which includes the mediator preparing a detailed report, including recommendations for both parties. None of these processes have happened yet."
If the strike goes ahead it will be the first national action by postal workers since the Royal Mail was privatised.
The union is campaigning against plans to close a final salary pension scheme as well as other issues such as pay and jobs.