Royal Mail has won a legal injunction blocking the first mass walk-out by postal workers for eight years.
The Communication Workers Union (CWU) had planned a 48-hour strike, scheduled to start on 19 October.
But the High Court today sided with Royal Mail's contention that the proposed industrial action was in breach of a side agreement mandating further actions must first take place.
The ruling now puts industrial action on ice, forcing both sides to get back to the table and negotiate. The earliest the CWU will be able to strike is in seven weeks' time.
Royal Mail said it would make contact with the CWU "as a matter of urgency".
CWU general secretary Dave Ward said: “We believe we can still take action before Christmas."
Shares in Royal Mail edged up 0.5 per cent in the minutes after the court decision.
Last week, the CWU notified Royal Mail of its intention to strike for the first time since the 501-year-old firm was privatised in 2013. The last mass industrial action among postal workers was in 2009.
Royal Mail responded to the announcement by calling the action "illegal". It subsequently filed an injunction on Monday with the High Court.
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.
With a union needing to give two weeks' notice of action, this puts an additional seven weeks into the process.
Royal Mail argued this meant strikes could not go ahead until the condition of additional talks was fulfilled.
The union said mediators were present at previous talks prior to balloting members for action. Such representatives, it argued, fulfil the requirements laid in the side agreement.
Following today's court ruling Royal Mail issued a statement saying:
Royal Mail confirms that the High Court has today ruled that the contractual dispute resolution procedures under the Agenda for Growth must be followed before industrial action can take place. The court’s final injunction means that any strike action before the dispute resolution procedures have been followed would be unlawful.
We will now make contact with the CWU as a matter of urgency to begin the process of external mediation. The mediation process will take close to Christmas to be completed, and may be longer. The first step is selecting a mediator acceptable to Royal Mail and the CWU from a panel that was agreed by both parties under the Agenda for Growth.
The Agenda for Growth is a balanced agreement which gives significant protections to Royal Mail employees. In return, the CWU committed to an industrial stability framework with defined processes and strict timescales to resolve disputes. We want to use them to do just that.
We are very committed to working closely with the CWU in order to reach agreement as a matter of priority.
On the steps of the High Court, Ward said he was "extremely disappointed" at the outcome.
Ward added: "We say to Royal Mail: unless you are going to reach an agreement that satisfies all the issues that are contained within this dispute… then make no mistake whatsoever, you will have to confront the harsh reality that industrial action and strike action will take place."
And deputy general secretary Terry Pullinger said:
This is just part and parcel of a modern-day dispute. This hasn’t, in reality, changed anything. We have to cancel two days but all it has done has deferred it.
If we get an agreement… then our ballot will have done the trick.