Jeremy Corbyn has "encouraged and endorsed" anti-Semites, a Labour peer has claimed.
Lord Winston said hostility to Jews has "infected the Labour Party so it's become endemic".
The scientist, who is Jewish, said Mr Corbyn spoke at a Labour meeting in the House of Lords on Wednesday and claimed he was "almost looking sheepish".
"He found it very difficult to articulate the words that were needed," he told BBC One's This Week.
The peer added: "I feel deeply ashamed of my party. Whether he likes it or not, Jeremy Corbyn has a lot to answer for. He has encouraged anti-Semites and he's endorsed them.
"In the modern world using social media, their conspiracy theories about Jews and statements about hate, spread like a disease and this virus is infecting the Labour Party."
The academic said he believes Mr Corbyn is a "good man" who is not anti-Semitic but warned he was "very worried about the quality of the leadership" because the firm action needed to tackle the problem had not been shown.
He claimed the Labour leader "doesn't seem to understand that his attitude actually decreases the chance of peace and raises the risk of violence".
Lord Winston warned that if Mr Corbyn stands by while the party "descends a disastrous path" he could make the party "unelectable".
Asked if Mr Corbyn was fit to be Prime Minister if he fails to tackle the problem, the peer replied: "I fear that he's not unfortunately and it troubles me to say that on air."