A lawyer and climate campaigner who revealed the Supreme Court’s decision on Heathrow’s third runway a day before it was made public has said going to jail for contempt of court “is a tiny price to pay to get the truth out there”.

Tim Crosland, director of environmental campaigning organisation Plan B Earth, faces jail for breaking an embargo on the court’s judgment allowing Heathrow’s appeal over controversial expansion plans in December.

Mr Crosland, who previously worked for the Serious Organised Crime Agency, described breaking the embargo as “an act of civil disobedience”, adding: “I have no choice but to protest the deep immorality of the court’s ruling.”

On Monday, three Supreme Court justices are hearing an application on behalf of the attorney general, Michael Ellis QC, to hold Mr Crosland in contempt of court, which is punishable by up to two years in prison and/or an unlimited fine.

Speaking outside of the Royal Courts of Justice in London before the hearing, Mr Crosland, who arrived at the court with a small suitcase, said he was prepared to go to jail and had packed his toothbrush just in case.

He told the PA news agency: “The Government knew the consequences of breaching that limit would be devastating for all our young people, for the global south, and they kept that information away from public view in order to smooth the progress of the £14-billion project to expand Heathrow Airport, and so I decided to blow the whistle.”

Opening the case for the attorney general, Aidan Eardley told the court that the Supreme Court’s decision “spread like wildfire” after Mr Crosland broke the embargo.

He added that Mr Crosland “ignored” a request by the Supreme Court to remove his statement from Twitter, and has “remained entirely unapologetic” since.

Mr Eardley said Mr Crosland had shown “wilful defiance to the authority of the court”.

Mr Crosland, who is representing himself, told the court that he broke the embargo on the Supreme Court’s judgment “because I believed that Heathrow expansion would breach the Paris temperature limit of 1.5C”.

He said that he believed this information “was being deliberately suppressed from the public domain”, so he decided that “the antidote to that suppression was the spotlight of publicity that would follow from breaking the embargo”.

Mr Crosland accused the Government of a “cover-up” of the consequences of allowing Heathrow to expand, a project “in which so much economic and political capital had been invested”.

He told the court the former transport secretary, Chris Grayling, “knew Heathrow expansion was inconsistent with” the terms of the Paris Agreement.

Mr Crosland argued that, when he received the draft judgment, “it seemed to me more or less inevitable that the Supreme Court was going to issue a judgment that compounded the Government’s original suppression of evidence”.

He concluded: “My sole intention was to counter the Government’s interference with the democratic process, by the deliberate suppression of vital information concerning the dangers of Heathrow expansion.

“The attorney general prosecutes me for highlighting the Government’s dishonesty and climate hypocrisy in the year of Cop26.

“It’s the classic case of retribution against the whistleblower by those attempting to conceal their own guilt.”

He asked the Supreme Court to “consider the consequences” of allowing Heathrow expansion, saying it would cause “the loss of life, the betrayal of the younger generation, the betrayal of the international community”.

Mr Crosland added that, “to all sensible people, to all people of conscience, and to all faithful public servants”, revealing the court’s decision was “trivial by comparison”.

In December, the Supreme Court overturned the Court of Appeal’s previous ruling that the Government had failed to take account of its own climate commitments when it approved the scheme for a third runway at Heathrow.

The UK’s highest court circulated a draft of its judgment to various parties involved in the case, including Mr Crosland, on December 9.

On December 15, the day before the ruling was delivered, Mr Crosland revealed the Supreme Court had allowed Heathrow’s appeal and said he was breaking the embargo in protest at the outcome.

The hearing before Lord Lloyd-Jones, Lord Hamblen and Lord Stephens is expected to conclude on Monday.

It is not yet know if a decision will be made on whether Mr Crosland is in contempt of court on Monday and, if so, what sentence will be imposed.