Jeremy Corbyn has been accused by a Jewish leader of failing to take anti-Semitism seriously - but the Labour leader promised an "immediate" investigation of any allegations.
Board of Deputies president Jonathan Arkush attacked Mr Corbyn's "deeply disturbing" response to a tweet by his brother criticising a Jewish Labour MP.
He said Mr Corbyn could not be "inert" where abuse in the party was alleged.
Mr Corbyn said: "Anti-Semitism is vile and wrong as is any form of racism."
Labour is currently investigating allegations of anti-Semitism including in its Oxford University branch, and last month it suspended one of its members for posting anti-Semitic tweets.
After Labour MP Louise Ellman said more needed to be done and that cracking down on such behaviour was "not just about words", Mr Corbyn's brother Piers tweeted saying this was "rubbish".
He added: "All #Corbyns are committed #AntiNazi. #Zionists cant cope with anyone supporting rights for #Palestine".
According to The Sun, when asked about his brother's tweet, Mr Corbyn said: "My brother has his point of view, I have mine and we actually fundamentally agree - we are a family that were brought up fighting racism from the day we were born."
Mr Arkush said: "Jeremy Corbyn's defence of his brother's belittling of the problem of anti-Semitism is deeply disturbing.
"We cannot imagine that any other minority's concerns would be dismissed off-hand in this way."
He said there had recently been "a stream of clear-cut cases of anti-Semitism in the Labour Party", and that the Jewish community as well as Labour politicians were "crying out" for the leader to take action.
"It would be incomprehensible for Mr Corbyn to remain inert and refuse to take this form of racism in his party seriously," he added.
London mayoral candidate Sadiq Khan recently said Mr Corbyn needed to take a "tougher stance" on tackling anti-Semitism, which he said should be a "a badge of shame" in the Labour Party.
Speaking on a visit to Norwich, Mr Corbyn said: "If anyone reports any form of anti-Semitism within our party, it is investigated immediately and cracked down upon.
"That message is unequivocal - we are a multifaith, multicultural Britain - let's respect each other and move on from there."