The Acorn union has launched an internal investigation and apologised to the Jewish community after a person attempting to join its Manchester branch was sent two Instagram messages saying “no time for Zionists” and “we are a pro-Palestine organisation”.

Acorn bills itself as “a community-based union of working class people — tenants, workers, residents”, which fights to give low-income communities a better life.

The man who attempted to join is Jewish and when he received the rejection messages, told his friend Stephane Savary, the national vice-chair of the Jewish Labour Movement (JLM).

Mr Savary challenged Acorn on Twitter, adding that not only had those who sent the rejection known his friend was Jewish, but that they had gone on to block him on Instagram.

The Jewish applicant to Acorn uses his Hebrew name on Instagram, writes in Hebrew and describes himself in his bio as “full-time” Jew”, and “new immigrant in the diaspora”.

On Monday the union issued a profound apology “for the upset and offence caused” to its Jewish members and the wider Jewish community.

Acorn said it had not yet identified the person who sent the messages, which it described as unequivocally antisemitic.

“In no way do they represent the views of our organisation”, the union said in a statement, adding that it was taking the opportunity to overhaul its social media practices as a result of the issue.

All members of Acorn UK with access to the Instagram account have been suspended while its investigation takes place, the union said, adding that “it is clear that there has been a lapse in our social media access practices”.

The union refused to say any more while the internal investigation was under way.

JLM said the incident had damaged the reputation of the good work the union is doing and called for the immediate expulsion of the person responsible for the Instagram messages.