Royal Mail Group has said it will use legal action to prevent its workers staging a 48-hour period of strike action later this month.
The strike is set for 19-21 October, after talks between the Communication Workers Union (CWU) and Royal Mail bosses failed to reach a conclusion on issues related to pensions, pay and jobs.
Royal Mail Group said it was "very disappointed" by the decision and planned to use legal action if the dispute could not be resolved.
Royal Mail will now invoke the legal obligation for both sides to go through external mediation, but threatened to take legal matters to the High Court to gain an injunction to prevent what could be the first Royal Mail strike since 2009.
A spokesperson for Royal Mail said: “We believe any strike action before the dispute resolution procedures have been followed would be unlawful strike action.”
CWU members voted in favour of strikes on Tuesday, with industrial action planned for the Christmas period.
The dispute relates to the closure of Royal Mail's final salary pension scheme, which it says will cost £1.3bn to keep open. It has proposed an alternative that guarantees members get all the contributions put in during their working life. It adds “discretionary increases” based on investment performance.
The group also wants to make changes to working schedules in order to compete with next-day delivery firms such as Amazon and Hermes.