The Countdown presenter Rachel Riley and former EastEnders actor Tracy Ann Oberman are preparing legal action against up to 70 individuals for tweets relating to their campaign against antisemitism in the Labour party, according to the pair’s lawyer.

Mark Lewis, who made his name representing phone-hacking victims, said he is contacting people who have either posted allegedly libellous claims about his clients or repeatedly sent them large numbers of messages, which he says is tantamount to harassment.

The lawyer also said he will go to court and force Twitter to release the details of individuals who made the contentious posts if users do not voluntarily comply with his request to provide formal contact details.

“This is a very early-stage legal procedure in order to identify the potential defendants,” Lewis said. “People need to realise that when they publish on Twitter they become a publisher. Whether it’s an initial publication or republication does not matter,” he said, emphasising that a retweet is grounds for a case.

Former EastEnders actor Tracy Ann Oberman. Photograph: Sarah Lee/The Guardian

He said he was approached by Riley and Oberman some time ago, following his success representing the food writer Jack Monroe in a legal case against Katie Hopkins, as well as his repeated criticism of Jeremy Corbyn’s Labour party.

Both Riley and Oberman have criticised Labour persistently and have in return faced substantial online abuse, with Riley being given extra security when appearing on Countdown.

“This is not about money,” Lewis said of his clients. “They’re not looking to enrich themselves by taking legal action. They’re looking to stop vile lies.”

Lewis said there was no guarantee legal action would be brought against all the Twitter users he had contacted, many of whom self-identified online as Labour supporters.

One of the Twitter users describes herself as a 17-year-old girl, earning Lewis criticism from those who accuse him of targeting a child. He said if the user could prove she is a minor then no action will be taken against her.

One pseudonymous Twitter account responded to Lewis’ request for his contact information: “Your attempts to silence me with threats and intimidation will not work. I will never stop speaking out against the barbaric treatment of the Palestinians by the viciously racist apartheid state of Israel. You, Oberman and Rachel Riley are pathetic. Now fuck off.”

Lewis has been a repeated critic of Labour’s treatment of claims of antisemitism. At the end of last year the lawyer was fined £2,500 by the Solicitors Disciplinary Tribunal after it concluded he acted with a lack of integrity when he sent offensive tweets to his opponents, including one which said: “Happy to celebrate your death too. I have not got time for your hideous evil.”

Lewis had argued he had been bombarded with hundreds of antisemitic messages and was responding “robustly to intimidating racist death threats”. At the end of last year he and his partner moved to Israel, citing the level of antisemitism in Europe.