A human rights watchdog has launched a formal investigation into whether the Labour Party "unlawfully discriminated against, harassed or victimised people because they are Jewish".
The Equality and Human Rights Commission (EHRC) said it had contacted Labour after receiving a "number of complaints" about allegations of anti-Semitism within the party, and had "carefully considered" their response before opening the probe.
The EHRC said the investigation would seek to determine whether "unlawful acts have been committed by the Party and/or its employees and/or its agents" and if the party has "responded to complaints of unlawful acts in a lawful, efficient and effective manner".
Labour said it would "cooperate fully" with the EHRC, and rejects "any suggestion that the party does not handle anti-Semitism complaints fairly and robustly".
A party spokeswoman said: "Labour is fully committed to the support, defence and celebration of the Jewish community and is implacably opposed to anti-Semitism in any form.
"We reject any suggestion that the Party does not handle anti-Semitism complaints fairly and robustly, or that the Party has acted unlawfully, and we will continue to cooperate fully with the EHRC.
"We support the efforts of the EHRC to draw attention to the obligations all political parties have under the Equality Act. But its ability to do so has been undermined by a 70% budget cut since 2010.
"Labour is the party of equality and in government we will strengthen the powers and functions of the commission.
"There has been a deeply worrying rise in anti-Semitism 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."