The Equalities and Human Rights Commission has launched an inquiry into antisemitism in Labour today.
The equality watchdog said it believes Labour may have “unlawfully discriminated against people because of their ethnicity and religious beliefs”.
It is now contacting Jeremy Corbyn’s party to set out its concerns and request a response.
If that response is found to be unsatisfactory, the EHRC could launch a formal investigation.
In a statement, an EHRC spokesman said: “Having received a number of complaints regarding antisemitism in the Labour Party, we believe Labour may have unlawfully discriminated against people because of their ethnicity and religious beliefs.
“Our concerns are sufficient for us to consider using our statutory enforcement powers.
“As set out in our enforcement policy, we are now engaging with the Labour Party to give them an opportunity to respond.”
The probe follows a complaint from the Jewish Labour Movement and Campaign against Antisemitism.
The last time a statutory probe was launched by the body was in relation to the Metropolitan Police.
If such an investigation is opened, the Commission could compel the party to reveal details of its handling of antisemitism cases including through internal communications like emails and texts.
It would also have the power to force the party to take on a plan of action to tackle hate.
Ruth Smeeth MP said in a statement: “This is a devastating indictment on the Labour Party. Decent members will be horrified that we have got into this position.
“However, after three years of appalling incidents of antisemitism from a small number of racists within our ranks which the leadership failed to tackle, this is a necessary step.
“I welcome the EHRC intervention today but today is another dark day in the history of our party which could and should have been avoided if concerns raised had been heeded last year.”
John Mann, MP for Bassetlaw, tweeted: “Obviously there will be resignations from those in power as this fully unfolds.
“Everyone should let the EHRC do its job ( as the Labour Government legislation empowering it requires).”
Barking and Dagenham MP Margaret Hodge wrote on Twitter: “faith in Labour’s complaints process is at rock bottom.
“It’s essential the EHRC make all necessary inquiries. We desperately need a culture of zero tolerance towards antisemitism in the Labour Party.”
Board of Deputies president Marie van der Zyl said: “This proposed independent investigation confirms what the Jewish community has known for a long time: that the Labour leadership has a problem with anti-Jewish racism which it is unable or unwilling to solve.
“We welcome this development and hope it will now focus minds on the extent and depth of antisemitism infecting the Labour movement”
A JLM spokesperson said: “We did not take that decision lightly. After years of anti-Jewish racism experienced by our members, and a long pattern of denial, obfuscation and inaction by those with the power and ability to do something about it, we felt there was little choice but to secure a fully independent inquiry, not encumbered by corrupted internal practices. Everything that has happened in the months since our referral supports our view that the Labour Party is now institutionally antisemitic.
“Our 99 year affiliation to Labour was born from the solidarity shown to our community by a Party that held social justice and equality as its core values.
“Democratic socialists stood side by side with the Jewish working class, and promised them a haven in Britain, free from the persecution they experienced in mainland Europe. Sadly, a century later that solidarity is lacking from all too many within the Party.
“This evening Jewish Labour members made clear that we will not unconditionally stand by whilst we are treated with such intolerance and contempt. We in our history have loved and respected the Labour Party too much to let this continue.”