Sally Rooney, Greta Thunberg and Brian Eno have written to the court of appeal in support of Palestine Action before next week’s hearing to determine the lawfulness of the ban on the direct action protest group.

The letter, composed of only six words – “We oppose genocide, we support Palestine Action” – is signed by more than 130 people and is the first time that prominent scholars, writers and activists have come together to defy the ban.

Three senior judges ruled in February that the ban on the organisation, the first on a direct action protest group under the Terrorism Act, was unlawful, but said that it should remain in place pending the appeal, which will begin at the court of appeal on Tuesday. After the high court’s decision, the Metropolitan police said it would stop arresting people, but has since resumed arrests.

Other prominent signatories include the writers Tariq Ali and China Miéville; the musicians Nadine Shah and Matt Black, a founder of Ninja Tune records; Judith Butler, a prominent feminist philosopher and queer theorist at UC Berkeley in the US; and the veteran environmental campaigner Jonathon Porritt.

By echoing the words of more than 3,000 people who have been arrested for alleged support of a terrorist group by holding placards saying “I oppose genocide, I support Palestine Action”, they too risk arrest.

Peter Hallward, a signatory and professor of modern European philosophy, at Kingston University, said: “This absurd and contradictory situation clearly cannot last much longer. And if enough people are prepared to defy such an incoherent and unjustifiable law, then we may be able to persuade the government that it’s neither legitimate nor enforceable.”

Rooney, the Irish author of Normal People and Conversations with Friends, cancelled a trip to the UK to pick up an award last year as she feared being arrested after she said she intended to use proceeds from her works to support Palestine Action. In a witness statement highlighting the ban’s impact on freedom of expression, she said her books could disappear from UK stores because of her support for the group.

Thunberg was arrested in December after holding a sign at a demonstration saying: “I support the Palestine Action prisoners. I oppose genocide.” Her placard was a reference to people on hunger strike at the time, who were in prison awaiting trial in connection with protests before the group was banned.

Some of the signatories signed earlier statements calling on the UK government to reverse the ban on Palestine Action and, after Thunberg’s arrest, expressing solidarity with the Palestine Action prisoners. But the letter to the court of appeal – signed by scholars from countries including the UK, France, Argentina, Australia, Canada and the US – goes further by flouting the ban.

Porritt said: “If you ignore what is happening in Gaza, the West Bank and East Jerusalem, you put your own humanity on hold. If you fight for the rights of all Palestinians, by supporting Palestine Action, hope can be reborn.”

Charles Secrett, who spent a decade leading Friends of the Earth in the UK, said: “Orwell warned that the first step towards tyranny is the abuse and misuse of language by an authoritarian government. Labour’s perverted interpretation of the word ‘terrorist’ in designating Palestine Action and its supporters as terrorists is a case in point.

“It is a travesty of justice and freedom of expression to equate peaceful, placard-carrying people of conscience, like students and pensioners, with violent extremists from groups like Isis [Islamic State] and al-Qaida.”

The letter has been posted on the website of Defend Our Juries, a civil liberties organisation, with a form that allows anyone to sign their name.

After the high court’s judgment, the home secretary, Shabana Mahmood, said she disagreed with its ruling “that banning this terrorist organisation is disproportionate”.