Sitting in his room in Parliament's Norman Shaw building, home to the suite of offices assigned to the Leader of the Opposition and his team, political adviser Harry Fletcher speaks calmly and softly.

His tone and patience belie the chaos in the corridors outside, where Labour leaders and MPs are embroiled in one of the most tumultuous episodes in the party's history.

A Westminster veteran with more than 25 years' experience of advising politicians, Mr Fletcher has just finished a 12-month shift assisting Jeremy Corbyn and Shadow Chancellor John McDonnell.

And in his long career as a political insider, he has never seen anything to match the disorganisation and failure that he witnessed while working with Mr Corbyn.

Mr Fletcher said the Labour leader had repeatedly ignored his calls for a greater effort to improve relations with the Jewish community.

For the Jewish vote to collapse is a huge warning

Mr Corbyn and his advisers also had a "deep-seated problem" with Israel which had led to the damaging effect on the relationship with British Jewry, the aide said.

Discussing his time advising Mr Corbyn and Mr McDonnell, Mr Fletcher painted a picture of shambolic scenes, with MPs' calls being ignored, media arrangements collapsing amid "crises after crises after crises", and thousands of emails going unanswered.

When Mr Corbyn was selected for last summer's leadership ballot, Mr Fletcher worked with him to highlight areas of policy - and personality - that might be problematic.

"I sat down with him in the very early days of his campaign to talk through where the Achilles' heels were, what issues he was likely to be attacked on, both personal and professional," Mr Fletcher explained.

"Top of the professional list was the perception that he was antisemitic, anti-Israel and that we needed to look at all the associations he has had in the past."

Mr Fletcher had a front-row seat as the Labour leadership repeatedly stumbled in its interactions with British Jews. He objected to the apparent plan last September to appoint a "Minister for Jews".

"If you go down the road for a Minister for Jews and the person appointed was, say, Luciana [Berger, the Labour MP], that would be appallingly tokenistic. It would have to be as part of a plan for dealing with all forms of discrimination and hate," he said.

A proposal to launch an advisory group of Jewish community representatives to assist Mr Corbyn disappeared without trace, Mr Fletcher claimed.

When a JC poll revealed in May that just 8.5 per cent of British Jews would vote Labour, and two-thirds of the community said Mr Corbyn had not done enough to tackle antisemitism problems, Mr Fletcher was "horrified".

"It was a massive alarm bell. For the Jewish vote to collapse has to be a huge warning. There have been half a dozen opportunities to act, and they have not been taken."

Mr Fletcher, whose paternal grandfather was Jewish and fled pogroms in eastern Europe in the early 20th century to settle in Manchester, said he repeatedly encouraged Mr Corbyn to engage with the community - either by making speeches on antisemitism or in the Jewish media.

But his suggestions went unanswered. "I think it's just a reluctance to engage, maybe it's a fear of making mistakes and making matters worse."

While Mr Corbyn was a "principled, honest" politician, "there's certainly a view held by some people within his office that the state of Israel should never have been created. It's very difficult, if that's what you believe, to accept any dialogue."

Mr Fletcher said communication problems could have been because of "the inexperience of the team", but that at times there were "tens of thousands of emails" unread.

"I never got to the bottom of whether it was a deliberate refusal to engage with Jewish issues or whether it was part of the general disorganisation," he explained.

Mr Fletcher believed Mr Corbyn's current position was "completely untenable" as he could not "command the authority" of Labour MPs in the Commons.

But if Mr Corbyn were to win this summer's leadership election he must "properly engage with Jewish community groups, make sure the Chakrabarti report is properly implemented and respond swiftly to any issues that come up involving Jewish issues or individuals who have made antisemitic remarks," Mr Fletcher said.