A lawyer and climate campaigner who leaked the result of a supreme court ruling on the Heathrow airport expansion has lost an appeal against a contempt of court finding.

Tim Crosland, the director of environmental campaign group Plan B Earth, was found in contempt of court for prematurely revealing the court’s decision on Heathrow’s third runway 22 hours before it was made public in December 2020.

The lawyer, from Elephant and Castle in south London, previously described breaking an embargo on the judgment as “an act of civil disobedience”.

In May, three supreme court justices found Crosland in contempt of court for his “deliberate and calculated breaches” and fined him £5,000, as well as ordering him to pay costs of £15,000.

Crosland challenged the contempt finding at a hearing in October, arguing that the judges failed to consider his “intentions, motivations and belief” when breaching the embargo.

He said he broke the embargo to draw attention to the government’s use of a “historic” 2C temperature limit above pre-industrial levels, rather than the current 1.5C target required by the Paris agreement, when considering the airport’s expansion.

His appeal was heard by the only five supreme court justices who were not involved in either the original contempt hearing or the Heathrow ruling.

Aidan Eardley, representing the attorney general, said at the October hearing that Crosland’s leak was a “publicity stunt” and argued that the campaigner could have publicised his views after the judgment was released.

Crosland said he believed that breaching the embargo gave him a better chance of bringing his points to the public’s attention and “sounding the alarm loudly”.

But in a ruling on Monday his appeal was rejected by all five judges, who concluded he could have expressed his views after the judgment was made public.

Lords Briggs, Kitchin and Burrows and Lady Rose said: “We have seen no persuasive evidence that Mr Crosland would not have been able to get his message across if he had complied with the embargo and refrained from discussing the outcome of the Heathrow appeal and his criticisms of the judgment until after it had been handed down.’’

Lady Arden, who gave a separate ruling, said Crosland’s conduct was “aggravated” by the fact that he is a qualified barrister and therefore would have been “particularly aware of the seriousness of not complying with the court’s directions”.

She added: “A barrister plays an essential role in the administration of justice. He owes duties to the court.”

In a statement after the ruling, Crosland said he plans to take his case to the European court of human rights.

He said: “It’s no great surprise that the supreme court has decided to side with itself, in continuing support of the carbon economy, which is driving us all to destruction.

“The government knows Heathrow expansion will cause the 1.5C Paris temperature limit to be breached, with devastating consequences for us all. But it has concealed that information from the public to pave the way for the £14.5bn project of its corporate sponsors.

“They are the real criminals, not those of us taking action to defend our communities and the people that we love.

“Retribution against the whistleblower is wrong. The case concerns the right to freedom of expression, which is fundamental to any democracy. I will now appeal to the European court of human rights in Strasbourg.”