Royal Mail has said it will take legal action in an attempt to stop thousands of workers walking out this month.
The postal firm said it would apply for an injunction at the High Court to prevent the industrial action.
Royal Mail workers are set to strike for 48 hours from 19 October in protest over pensions, wages and jobs.
The company said the strike would be “unlawful” if the Communication Workers Union (CWU) did not follow dispute resolution procedures.
Royal Mail had given the postal workers trade union until midday to withdraw the strike threat. After the union refused, Royal Mail said it would go ahead with filing the court application.
The industrial action follows an 89.1% vote in favour of a strike last week following a ballot of the CWU’s 110,000 members.
The union said the company’s move to reform workers’ pensions meant its members would lose up to a third of their retirement entitlements.
CWU general secretary Dave Ward has called it a “watershed dispute” that will determine the future of the postal service.
The CWU vote, which had a 73.7% turnout, was the first major ballot since the introduction of the Trade Union Act, which requires a 50% turnout.
The union did not rule out further strike action in addition to the 48-hour walkout later this month.
CWU deputy general secretary Terry Pullinger has said the two sides have been in negotiations for 18 months.