The Equality and Human Rights Commission has launched a formal probe into claims of antisemitism within Jeremy Corbyn's Labour party.
The investigation will seek to establish whether Labour has discriminated against, harassed or victimised anybody because they are Jewish.
The commission launched the probe after "receiving a number of complaints about allegations of antisemitism" within the party.
"We have carefully considered the response we received from the Party and have now opened a formal investigation under section 20 of the Equality Act 2006 to further examine the concerns," the commission added.
It said the Labour party has vowed to "cooperate fully" with the investigation.
A report of the body's findings will follow the probe, potentially including recommendations.
A Labour Party spokesperson said: “We reject any suggestion that the Party does not handle antisemitism complaints fairly and robustly, or that the Party has acted unlawfully, and we will continue to cooperate fully with the EHRC.”
They added: “There has been a deeply worrying rise in antisemitism in the UK and across Europe. We are taking action to root it out of our party by strengthening our rules and procedures.
“But the issue can only be properly dealt with by all political parties working together to protect the interests of the Jewish community and to combat racism in politics, the media and in society more broadly.
“That includes the need for the Conservatives and other parties taking action to deal with racism in their own ranks.”
The watchdog said it will be carrying out a two-pronged investigation into whether Labour, its staff or agents have committed antisemitic crimes and whether the party has addressed complaints "in a lawful, efficient and effective" way.
That latter focus will examine if Labour's party rule book and investigatory and disciplinary processes are sturdy enough to deal "efficiently and effectively" with discrimination complaints.
It will also check what steps Labour has taken to carry out recommendations in the Chakrabarti Report, and by Baroness Royall and the Home Affairs Select Committee.