Bosses argued in court that the Communication Workers’ Union had fallen foul of the law by not going through dispute resolution procedures agreed at the time of privatisation. Union reps denied this claim.
Granting the injunction, Mr Justice Supperstone said: "I consider the strike call to be unlawful and the defendant is obliged to withdraw its strike call until the external mediation process has been exhausted."
CWU said it is "extremely disappointed" at the ruling and vowed to call further strike action unless a satisfactory agreement with Royal Mail is agreed once the mediation process has been completed.
General secretary Dave Ward said: "The company is deluded if it believes its court room politics will resolve this dispute.
“Instead the company’s actions will have the complete opposite effect.
"Postal workers’ attitude towards the company will harden and it makes us more determined than ever to defend our members’ pensions, jobs, service and achieve our objectives.
“Unless the company significantly shifts its position on a range of issues and we can quickly conclude a good agreement for our members then strike action is inevitable."
He stressed that the union walked into court today with a massive 90 per cent yes vote for strike action and walked out of court with that mandate intact.
"We want an agreement and will comply with the injunction to undertake further external mediation," Mr Ward said.
"But sooner rather than later Royal Mail Group will have to confront the harsh reality that it is completely out of touch with the views of its workforce.“
Workers at Royal Mail, which was privatised by the Conservatives and Lib Dems in 2013, are up in arms about the proposed closure of the current defined benefit pension scheme.
They have proposed their own alternative pension programme for a “wage in retirement” and are also seeking to protect terms and conditions and shorten the working week to 35 hours.
The CWU brought four live chickens in a mail cage down to the Royal Courts of Justice to await the verdict. Reps said the birds represented Royal Mail bosses Moya Greene, Steve Cameron, Jon Millidge and Sue Whalley – who they said were running scared of negotiations by seeking an injunction.
CWU London divisional rep John Simkins told the Star: “It’s a disgrace [Royal Mail bosses] feel they have to respond by going to the courts. We’ve had a massive turnout and a massive yes vote. They’ve had 18 months to reach an agreement, and it wasn’t for want of trying on our part.
“It’s trying to use the law against its own employees. They should be ashamed of themselves.”
But he said it was no surprise that bosses were using the courts in this way. “The whole legal system is just designed to thwart working people,” he added.
The demonstrators were joined outside the court by Labour’s shadow post and steel minister Gill Furniss, who blasted: “[Royal Mail] may be working for Moya Greene and her £2 million-a-year pay package, but it’s not working for the workers.
“You can be assured that Labour are behind you every step of the way.”
Campaign for Trade Union Freedom chairman Tony Burke told the Star: "This is unbelievable given the size of the vote in favour and a turnout that passed the 50 per cent mark by miles.
"It's ludicrous that the High Court has granted the injunction."
A Royal Mail spokesman said: "We will now make contact with the CWU as a matter of urgency to begin the process of external mediation. We are very committed to working closely with the CWU in order to reach agreement as a matter of priority."