Union members had overwhelmingly backed a call for strikes over pensions and pay, with the 48-hour walkout then set for next Thursday.
A spokesman for Royal Mail said the court's final injunction means that any strike action before the dispute resolution procedures have been followed would be unlawful.
'Legal proceedings and Royal Mail's court room politics will not solve the company's problems.
Royal Mail on Monday applied to the High Court for an injunction to prevent any strike action before an agreed mediation process for settling labour disputes had been exhausted.
'We will now make contact with the union as a matter of urgency to begin the process of external mediation.
"Postal workers delivered a massive "Yes" vote for strike action and we are determined to take whatever steps are necessary to deliver an agreement that will protect and enhance our members' terms and conditions and improve the range of services on offer to customers", he said.
But CWU deputy general secretary Terry Pullinger slammed Royal Mail for "playing politics" rather than engaging with the union to reach an agreement. In return, the CWU committed to an industrial stability framework with defined processes and strict timescales to resolve disputes.
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. We want to use them to do just that.
The CWU had balloted 110,000 members, with 89.1 per cent supporting strike action on a 73.7 per cent turnout.
Update: 17:50: Here's a video from Royal Mail explains that it's business as usual as negotiations continue.
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