An open letter signed by 1,000 supporters of the Jewish Labour Movement has been published in the Guardian, condemning the decision not to expel Ken Livingstone over his comments on Hitler.

Registering their “disgust and frustration” at the outcome of this week’s disciplinary hearing into Mr Livingstone’s conduct, the party supporters said "enough is enough".

Signatories include MP Luciana Berger, Nicky Gavron, the former deputy mayor of London, and dozens of Jewish Labour councillors and activists from across the country.

Iain McNicol, Labour’s general secretary, confirmed on Thursday that a new investigation is now under way into Mr Livingstone’s comments since his hearing concluded on Tuesday evening.

Mr McNicol emailed members of Labour’s national executive committee and said “many complaints” had been received by the party since the national constitution committee decided to suspend, rather than expel, Mr Livingstone.

“I have instructed my staff to follow the procedures and begin an investigation into whether the party’s rules may have been broken again,” Mr McNicol wrote.

“If they have then I am determined they are investigated fully and properly.”

He will update members at the next NEC meeting.

Mr Livingstone has repeated various comments about Hitler and Zionism, as well as alleging Jews collaborated with Nazis. He has also blamed the Jewish media and alleged the complaints against him are designed as attacks on Jeremy Corbyn, the party leader.

The letter in the Guardian states: “The decision to allow him to remain a member presents us with an immediate dilemma about our future in the party

“Despite pledges of ‘zero tolerance’ on antisemitism, Labour has been found wanting when it truly matters. The Jewish community has a proud history with Labour, but this decision has thrown its future into jeopardy.

“We are sick of the complacency shown towards the prejudice we face. Enough is enough.”

Meanwhile, Peter Mason, a Jewish Labour councillor who sits on the NCC but did not take part in Mr Livingstone's case, has condemned the outcome of the hearing and said his party was "running out of second chances with the Jewish community".

Mr Mason, writing for the Labour List website, said there was "no way" he could have sat on the panel for Mr Livingstone's hearing.

"In dealing with some of the most difficult disciplinary cases the Labour party handles, the expectation on NCC members is very clear: honesty, transparency, objectivity," he wrote. 

"Deal with the facts of the case, and don’t pre-judge the outcome. Listen to the arguments. Act in the best interests of the party. If you are conflicted, or pre-determined in your view, do not put yourself forward to hear a case.

"On Livingstone, this wouldn’t have been possible for me. There was no way I could have taken part. Time and again Livingstone has sought to offend British Jews.

"I have seen first-hand his longstanding belligerent and offensive behaviour toward the Jewish community, which has had a lasting and damaging impact."

Mr Mason expressed his concern about the make-up of the panel, and said the "slap on the wrist" given to Mr Livingstone was "not a proportionate punishment".

The full JLM letter published in the Guardian:

We are former and current Labour members and supporters, from across the Jewish community and all sides of the party. We may disagree on policy, both domestic and international, but we are united by our unwavering commitment to anti-racism.

We collectively feel a sense of disgust and frustration at the decision by the NCC to not expel Ken Livingstone from the party. Livingstone’s comments betray a party that was founded on the values of equality and inclusivity. His history of inflammatory remarks against our community – be it his suggestions that our community’s “wealth” determines our vote, or his recent smears of victims of the Holocaust – surely have no place in a progressive party.

The decision to allow him to remain a member presents us with an immediate dilemma about our future in the party. Despite pledges of “zero tolerance” on antisemitism, Labour has been found wanting when it truly matters.

The Jewish community has a proud history with Labour, but this decision has thrown its future into jeopardy. We are sick of the complacency shown towards the prejudice we face. Enough is enough. We fully support the Jewish Labour Movement’s proposal to initiate a debate at Labour party conference in September 2017 promoting the expulsion of Livingstone from the party. We also support calls for an immediate review by the NEC of the decision.

Lastly, we would like to thank those in the Labour party who have offered us messages of solidarity, and would urge those who disagree with this decision to call on their representatives to speak out against it publicly.

The full list of signatories can be viewed here.