Britain's equality watchdog said on Tuesday it was launching a formal investigation to determine whether Jeremy Corbyn's Labour Party has discriminated against, harassed or victimized people because they are Jewish.
Labour, Britain's main opposition party, has promised to co-operate fully with the investigation, it added.
"We contacted Labour after receiving a number of complaints about allegations of anti-Semitism in the party," the Equality and Human Rights Commission said in a statement.
"We have carefully considered the response we received from the party and have now opened a formal investigation ... to further examine the concerns."
Labour MP and Shadow Women and Equalities Secretary Dawn Butler said: "I think that it's a disappointing moment. I mean I feel some shame in the fact that we've even got any racists in our party. But we are a party of over a million people and racism is on the rise globally, and some of that is in our party and we have to rule it out."
She said it was "really important that the Labour Party builds trust within the Jewish community, and I think that this investigation will help us to do that."
Labour has been riven by allegations that the party has become hostile to Jews under Corbyn, a longtime supporter of the BDS movement against Israel. In 2012, Corbyn attended a conference with Hamas terrorists in Doha. His supporters accuse political opponents and right-wing media of misrepresenting his views.
In February, seven British lawmakers quit the party over its anti-Semitism problem.
