The vast majority (83%) of British Jews believe the Labour Party is too tolerant of anti-Semitism among its MPs, members and supporters, a poll suggests.
This compared with 19% for the Conservatives and 36% for the Liberal Democrats, according to the YouGov survey for Campaign Against Antisemitism (CAA).
The poll follows criticism of Labour's response to anti-Semitism in its ranks, although the party committed in its general election manifesto to "build a society and world free from all forms of racism, anti-Semitism and Islamophobia".
British Jews are also split on how well institutions protect them.
According to the survey, 65% believe the Government does not do enough to protect British Jews, while 52% think the Crown Prosecution Service could do more.
Sizeable minorities thought the police (41%) and the courts (47%) could do more, while only 39% of British Jews felt confident that anti-Semitic hate crime would be prosecuted.
The poll comes after the Community Security Trust, which monitors anti-Semitism in Britain, reported a record number (767) of anti-Semitic hate incidents in the UK in the first six months of 2017.
Despite this, the CAA/YouGov survey suggests anti-Semitism, measured by how many respondents agreed with seven anti-Semitic statements, has been in decline over the past three years, dropping to 36% in 2017
Gideon Falter, chairman of Campaign Against Antisemitism, said: "We now have data that show that in a very British way, fairly and quietly, Britons have been rejecting anti-Semitic prejudice.
"British society has shunned a growing worldwide addiction to anti-Semitism and proved that so-called British values are no mere buzzphrase, but are embedded in our national being.
"However, our research shows that one in three British Jews has become so fearful of mounting anti-Semitic crime and the failure to excise anti-Semites from politics that they have considered leaving Britain altogether.
"Our research clearly shows that British Jews have pointed their fingers at the Crown Prosecution Service and the Labour Party.
"If British society can fight anti-Semitism, why are our world-renowned criminal justice system and some of our famous political parties still doing too little?
"There is not a moment to lose. Without urgent change, British Jews may start to leave, as has happened elsewhere in Europe."
Former cabinet minister Sir Eric Pickles, of Conservative Friends of Israel, said: "Modern anti-Semitism has been allowed to flourish in the left of British politics, unchallenged by the Labour leadership, this report is a wake-up call.
"They cannot continue to be half-hearted in their approach, the time has come to root anti-Semitism out of British life."
A Labour spokesman said: "The Labour Party campaigns against anti-Semitism and condemns all anti-Semitic abuse.
"That's why Jeremy Corbyn set up the Chakrabarti Inquiry into anti-Semitism.
"Its recommendations have already led to far-reaching changes to the practices of the Labour Party.
"The party has also taken decisive disciplinary action over allegations of anti-Semitism and will continue to do so in the future."
:: YouGov surveyed 2,025 British Jews between July 19 and August 8.
Labour MP John Mann, who chairs the all party parliamentary group against anti-Semitism, said: "We know that British Jews are concerned and we are working to address those concerns.
"It is beholden on organisations to not sensationalise anti-Semitism but rather to work to put national frameworks in place to defeat it.
"We will continue to work with CST and other mainstream groups towards this goal."