Judge P. Kevin Castel, in Manhattan, said there was nothing “inherently improper” about using AI to help write a legal filing, but lawyers have a responsibility to ensure their accuracy.

Steven Schwartz — who has been fined alongside colleague Peter LoDuca and their law firm Levidow, Levidow & Oberman — apologised in May for his mistake.

Judge Castel said they “abandoned their responsibilities when they submitted non-existent judicial opinions with fake quotes and citations created by the artificial intelligence tool ChatGPT, then continued to stand by the fake opinions after judicial orders called their existence into question”.

In a statement, Levidow, Levidow & Oberman said: “We made a good faith mistake in failing to believe that a piece of technology could be making up cases out of whole cloth.”