One of the candidates to be the next Labour leader was a key supporter of a controversial Palestinian cleric who invoked the antisemitic blood libel.
Jeremy Corbyn has also previously described the terrorist groups Hamas and Hizbollah as "our friends".
Mr Corbyn was backed for the leadership by 36 Labour MPs this week and will compete with Andy Burnham, Yvette Cooper and Liz Kendall to become leader of the opposition in September.
Israel supporters in the party described Mr Corbyn's candidacy as "a joke" and "disgraceful".
The Islington North MP has been one of the most vociferous critics of Israel in Parliament. In 2012 he backed a call for an inquiry into the influence of "pro-Israel lobbying groups" on government.
He was a leading supporter of Islamic cleric Sheikh Raed Salah. He defended Salah during the Palestinian's high-profile appeal against a Home Office attempt to deport him in 2012. During the case, senior judges ruled that the cleric had invoked the blood libel in a 2007 speech.
As part of a group of MPs in 2012, Mr Corbyn invited to the House of Commons an organisation which had previously featured Hamas leader Ismail Haniyeh as a guest speaker. He previously said the government's decision to label Hamas as a terrorist group was "a big, big historical mistake".
Labour Friends of Israel director Jennifer Gerber said of Mr Corbyn's references to Hamas and Hizbollah: "These two organisations support terrorism [and] are virulently antisemitic. "These are views which Labour Party members rightly find utterly repugnant. I hope that, during this contest, Jeremy Corbyn makes clear that so does he."
Mr Corbyn was judged by Ofcom to have breached impartiality rules in 2010 when appearing on George Galloway's show on Iranian-backed Press TV . The leadership candidate had called for economic sanctions to be imposed on Israel.
Two years ago Mr Corbyn enjoyed a Shabbat dinner with members of the Jewish community in his constituency at the home of Chabad rabbi Mendy Korer.
In an open letter on Tuesday, Alan Johnson, senior research fellow at pro-Israel group Bicom, said that Mr Corbyn's support for Sheikh Salah and the terror groups was a "deal-breaker".