A former Archbishop of Canterbury has warned that Labour will not be ready to govern if Labour leader Jeremy Corbyn does not act to combat antisemitism.
Lord Carey of Clifton said he did not wish to suggest that the Labour Party was riddled with anti-Semitism.
But the problems were real enough that Corbyn had already promised to deal with the issue as a matter of urgency.
"If he does not take effective action this will demonstrate that the Labour Party is not ready to govern," said Lord Carey in a lecture, Combatting History's Oldest Hate - A Christian Perspective. "Antisemitic attitudes stubbornly persist in a few dark corners in Britain."
Lord Carey was speaking in Emmanu-El Synagogue in Manhattan in the US.
He was delivering the annual Dorothy Gardner Adler State of Anti-Semitism Lecture, endowed by Simon Wiesenthal Centre trustee, Allen Adler.
Lord Carey, Archbishop of Canterbury from 1991 to 2002, who is one of the patrons of the Simon Wiesenthal Centre's exhibition, People, Book, Land: The 3,500 Year Relationship of the Jewish People, said: "Whether anti-Semitism is the world's greatest hate may be a matter of debate, but there is no denying that in the western tradition we may trace its presence back hundreds of years."
It has taken many forms in Europe and Britain and is still "evilly present" today, Lord Carey continued.
He quoted Lord Sacks, former Chief Rabbi and most recent winner of the Templeton Prize, who has written: "In the Middle Ages, Jews were hated for their religion. In the 19th Century they were hated for their race. Today, they are hated for their nation state. Israel, now 68 years old, still finds itself the only country among the 193 in the UN whose right to exist is routinely challenged and in many quarters denied."
Lord Carey said there have been worrying signs in recent years. The Community Security Trust, a respected Jewish organization, reported in 2014 that antisemitic incidents in the UK reached their highest level in 30 years.
Lord Carey said: "What is often forgotten, or deliberately ignored, is the fact that Israel is the only democratic nation in the region and this should be the cause of celebration rather than rebuke. In the matter of a mere 68 years a tiny group of people has transformed a strip of land into a free place of hope. It shames its larger neighbours, most of whom are failed States. Israel eclipses all 22 members of the Arab League in scientific output and has more Nobel Prize winners than all of them put together."
He said Israel was the only nation in the Middle East where Christianity is not only not persecuted, but granted freedom of expression, freedom of worship and a place where Christians in the Middle East are safe.
He criticised the divestment and boycott movement.
The fact that Israel alone is so often singled out high profile campaigning against its products is an indicator of antisemitism, he said. Lord Carey asked: "Where do we find equivalent reaction against Saudi Arabia where Christian worship is banned? Or against Sudan where war against Christians and minorities has continued for decades? Or Iran, with its abuse of human rights?"
His attention had been drawn recently to antisemitic language in the Labour Party in England.
Lord Carey also said Islam could be part of the solution. "In countries such as France and the UK antisemitic actions are largely done by disaffected Muslims who have a particular hatred towards Jews. And, yet we know that most Muslims are good people, wanting the best for their children and themselves and desiring peace with their neighbours. We should not fall into the trap of believing that the awful activities of Islamic terrorists reflect the faith of the vast majority of Muslim believers. The opposite is the case. Here we find the dilemma for Muslim people. The savage ideology of groups like IS, Al Quaeda, Al Shabaab and others are shaped not be mainstream Islam but by the mistaken and warped theologies of leaders like Abu Bakr al Baghdadi.
"A moderate and reforming Islam must be enlisted as part of the solution and not the whole of the problem. Tough talking is only productive in the context of friendship and understanding."