The Equality and Human Rights Commission has launched a formal investigation into the Labour Party over allegations of anti-Semitism.
The watchdog told the party in March it had received a number of complaints and was considering its next steps.
It will now formally look into whether Labour has “unlawfully discriminated against, harassed or victimised people because they are Jewish”.
Labour has said it will “co-operate fully”, the EHCR added.
The party has been plagued by accusations of anti-Semitism since mid-2016.
The leadership has been accused of tolerating a culture of anti-Jewish prejudice by a number of its own MPs, some of whom have quit the party in protest.
Leader Jeremy Corbyn has insisted he is getting to grips with the issue and has beefed-up the party’s internal disciplinary procedures.