The Irish novelist Sally Rooney could be arrested under the Terrorism Act after saying she intends to use proceeds from her work to support Palestine Action, which was proscribed as a terrorist organisation in the UK last month, a legal expert has warned.

Meanwhile, No 10 said that supporting the group was an offence under the act, after Rooney had made her pledge.

Royalties from Rooney’s books, including Normal People and Conversations with Friends, along with BBC adaptations of them, would be used to support Palestine Action, she wrote in the Irish Times over the weekend.

The legal expert also said that the bestselling writer could face prosecution if she were to express her views at, for example, a UK book festival, underscoring the proscription’s “gross disproportionality”.

While the prime minister’s spokesperson would not respond to the author’s comments specifically, they said that there was “a difference between showing support for a proscribed organisation, which is an offence under the Terrorism Act, and legitimate protest in support of a cause”, according to the Press Association.