An adviser on racism hired to advise the Royal Court Theatre — which was accused last weekend of scheduling a play that included a grossly antisemitic stereotype — is herself in favour of an all-out boycott on Israel, the JC can reveal.

Shoomi Chowdhury, who until June headed a team from the anti-racism consultancy Sour Lemons, which advises the Royal Court, signed an open letter calling on Sainsbury’s to stop selling goods made in the Jewish state.

“These tactics worked against apartheid South Africa, and they’re starting to work against apartheid Israel too,” the letter said. “The call has always been explicitly and openly about boycotting Israeli-made goods.”

It has also emerged that the founder of Sour Lemons, Sade Banks, said in 2013 she had been researching Israeli companies to boycott.

She claimed there were so many firms to avoid that “it made me feel a bit sick”.

Earlier this month, actors and directors rounded on the top London theatre after it emerged that an upcoming production featured an unscrupulous billionaire called ‘Hershel Fink’.

The theatre apologised “unreservedly” over what it admitted was an antisemitic stereotype and said its writer has decided to change the name of the character — a protagonist — to “Henry Finn”.

The JC’s revelation that the Royal Court’s former lead adviser on racism has backed a boycott of Israel will add to the pressure on the theatre.

Responding to the outrage over Rare Earth Mettle, the play accused of antisemitism, Sour Lemons said it was “sad and sorry” and that it was “not involved in any artistic decisions at the Royal Court”.

Sour Lemons started an “anti-racism partnership” with the theatre in April this year. This involved the creation of two groups within the company: one “anti-oppression group” formed of white employees, and one “accountability group” for non-whites.

At the time, Ms Banks claimed the Sour Lemons model was unique because “by centring the needs of Black and Global Majority colleagues, it forces us to acknowledge that racism is happening over here”.

Royal Court artistic director Vicky Featherstone announced an “anti-racist reflection and action” plan in June, telling the Guardian it would address the plays staged by the theatre.

She said: “We have all inherited certain narrative structures. Some still stand but a lot of them need to be interrogated. Who are the people who shape the stories we tell? Who decides what a good play looks like? In the end, the biggest change will be in telling stories differently.” Sour Lemons did not respond to a request from the JC for further comment.

Reacting to the revelation about the play’s “Jewish character”, comedian David Baddiel questioned how anyone could fail to realise the character’s name was Jewish. Actor Olivia Mace said: “The character with that name resembles a negative Jewish stereotype. That wasn’t unconsciously done. This makes the play antisemitic.”

Sour Lemons did not respond to a request for comment.