He is credited with being one of the key figures behind Jeremy Corbyn's election as leader of the Labour Party - the driving force behind an increasingly influential hard-left movement.

So it is a surprise to find Jon Lansman sitting at a desk in a run-down office, where, instead of aides bustling around him, there are sparsely populated meeting rooms and eerily quiet corridors.

Mr Lansman is the founder of the controversial "Corbynista" pressure group, Momentum, which was set up to capture and retain the grassroots enthusiasm sparked by Mr Corbyn's campaign, but whose opponents fear will purge the party of moderates. It is becoming, they say, a "party within a party".

Having spent more than 30 years in the political shadows, relatively little is known about Mr Lansman. But in his first in-depth interview, he describes what happened behind the scenes in the astonishing leadership contest that saw Mr Corbyn take power, and how the "radicalism" of kibbutz life in Israel sparked a fervour in him that has fuelled his career.

"I was involved from the very beginning," he says. "I wanted to have a candidate from the left and I was actively encouraging people to stand. Various other people had declined and I eventually settled upon Jeremy as someone I wanted to persuade to stand."

Initially Mr Lansman, who is editor of the Left Futures blog, was not convinced that the Islington MP was the right man for the job, but he quickly changed his mind.

"Jeremy was not the first person you thought about as having the same conventional leadership qualities that the media and other Labour MPs think are necessary.

"But actually I came to see the reason he did well was because he didn't have those qualities. He had other things instead and people who made this happen were, if not members of the party, active politically - more than most."

Last summer's leadership contest was sparked into life when the JC challenged Mr Corbyn with a series of questions about his associations with Holocaust deniers, terrorists and antisemites.

Mr Lansman, 58, is relaxed about how he and his colleagues on Team Corbyn handled the episode.

"I do regret we weren't able to deal with the concerns in a more timely way," he says. "There was a missed opportunity to use the Jews in the team but we were operating with a small staff. We should have been more involved from the start, because we could have helped answer those questions."

Brought up in a "typical Orthodox family" in Southgate, north London, Mr Lansman first went to Israel aged 16 just after the Yom Kippur War to visit an aunt who had made aliyah.

"I worked on a kibbutz in the Negev and my aunt lived in Beersheva. It was actually a very politicising experience. When I did my barmitzvah I saw myself as a Zionist and I think after I went there I felt it less.

"I was more interested in the kibbutz and what I liked about it was the pioneering spirit, the sense of community and radicalism of it."

Is it the spirit that he witnessed in 1970s Israel that now fuels Momentum - seen by many as a hotbed of anti-Zionist fervour on Britain's political left?

Mr Lansman does not tackle the point head on, but explains: "I think Labour politicians look as if they all come from the same mould, you don't have the diversity you used to have, people with real experience of life.

"People come out of university and treat politics as a career and it means you end up with people who look and sound the same. It is damaging for the Labour Party. Jeremy offered something different. He is getting real people involved again."

The failure to respond to the Jewish community's concern about Mr Corbyn's ties to Holocaust deniers was down to "practical" reasons, Mr Lansman says.

"One of the problems is the questions related to meetings that happened before electronic records. You have to check the correspondence on paper and it can take many days to do the necessary checks. It makes it hard to respond instantly because you haven't got the evidence."

Distinctions should have been drawn, he believes, between how Mr Corbyn responded to such issues as a backbencher, and as the leader of the party. He can no longer dodge responding simply to avoid "satisfying his critics". Now there is a responsibility to react, and the Corbyn camp missed the boat last summer.

"Perhaps we were too slow to move and recognise that as a candidate for leader you have to respond to those things."

But Mr Lansman says there is no reason why Zionist Labour supporters cannot find a place in the Corbyn Labour Party.

"I have Zionist friends in the party. Jeremy supports the existence of Israel, he wants peace and co-existence. Why should Israel supporters not have a place in Labour? Of course they should. I've been arguing for two states long before it was acceptable within the Jewish community to argue for two states.

"I remember arguing with my great-aunt when I was 13 that there were Palestinians and they should have a homeland.

"What we are saying will strike a chord with people in the community and we absolutely need to mend and build bridges. For me it is a priority and that is why I am talking to the JC.

"Yes, of course the vast majority of British Jews are supportive of Israel as a Jewish state - and actually so is Jeremy - but they are far from supportive of all aspects of what is currently happening there. The Labour Party has to be concerned with a broad view, and the pursuit of peace.

"I don't think you can fault Jeremy on his concern for peace. He is not a warmonger, he doesn't want killing and death.

"I think Jews in Britain want peace too. I think Jeremy's message of fairness for the Palestinians is not something that will be rejected by the Jewish community."

Mr Lansman believes he has a grip on how Jews feel - partly through his regular reading of this newspaper. He says he uses the iPad edition to "keep up with what happens communally and elsewhere" every week.

"The other important thing is on all of the other issues - on antisemitism, kashrut - Jeremy offers something to the Jewish community. He is totally committed to things Jewish people have a natural sympathy for, so it is quite frustrating that Israel and Palestine has to dominate."

He acknowledges that it is going to be "an uphill struggle" to carry the community with Labour but says: "I think we will take a significant number if we can present the case in the right way."

Mr Lansman criticises communal leaders for failing to represent secular Jews: "The community leadership are really not speaking for the whole community in all sorts of ways. It's too influenced by religious elements and that's not healthy.

"They are not speaking for secular Jews. There is no real place for us, we don't have much of a say, and are not represented by the Board of Deputies." However he accepts there is still "work to do" to improve Labour's relations with community leaders.

But how would he challenge the antisemitism that has seeped into hard-left politics? "I criticised Ken Livingstone for writing off the Jewish community because he thought they were 'wealthy' so they wouldn't vote for him."

"It is still the case that Jews in Britain are socially more progressive than their non-Jewish neighbours with the same socioeconomic status.

"I don't think he was antisemitic but it was just wrong. It showed a complete misunderstanding of the community. He was sloppy with his language on Jews and Zionists. You have to be very careful. I'm critical of anyone on the left who does that."

He defends Momentum's decision to join with the Stop the War Coalition to lobby MPs to vote against the bombing of Daesh in Syria, but admits the anti-war group had been "stupid" for blaming the Paris terror attacks on the West and allowing online bullying and threats to take hold.

"I think even Stop the War themselves have accepted they were stupid for publishing those articles. There were some very silly things published, and wrong things. I'm completely against trolling people and abusing people on Twitter and social media.

"I think the level of abuse has been exaggerated and I don't think it comes from Momentum supporters. Maybe one or two, but generally I don't think so."

Mr Lansman has worked for Tony Benn and Michael Meacher, both prominent figures on Labour's left, but for years he was in the political wilderness.

In December, Labour Friends of Israel vice-chair Rebecca Simon picked up on the apparent secrecy around him, telling a Limmud conference audience that other than the fact he was Jewish and "Jeremy's friend", no one knew anything about him.

After so many years as a backroom adviser and co-ordinator, how does he now cope in the media spotlight?

"I've never talked about myself before," he says. "I've never sought public attention.

"Being thrust into the limelight is hard. There have been news pieces about me and my family which have been based on lies and distortion.

"I am perhaps a legitimate target, but my family, why? It is just dreadful."

The father-of-three explains his penchant for the backroom as being in part due to his wife's death from breast cancer 17 years ago. "Not long after our third child was born, my wife Beth went to the hospital with a lump. Three years later they had failed to diagnose it. She had a tumour the size of a tennis ball.

"My youngest was eight when Beth died. It was tough but you do learn to cope. That experience really made me lose my interest in the world of work.

I didn't feel much ambition and that was why I sought a backroom mentality."

The Lansman family still found humour in those dark times; when non-Jewish Beth took to wearing a wig during chemotherapy treatment, a rabbi mistook it for a sheitel.

Mr Lansman says he marks Jewish festivals such as Pesach and Chanucah with family events, but struggled to tell his parents that he no longer goes to shul on Yom Kippur.

Rabbi Jonathan Romain, of Maidenhead Synagogue, had offered to help Beth complete a conversion, but the Lansmans dropped the idea when it was clear Jon would also have to take part in lessons.

"As an atheist, it did not feel right to put her through a conversion when I wasn't committed to it," he explains.

He tells me that his father became a Conservative councillor in Hackney in part as a rebellion against his son joining Labour - and was so well-respected that on his death he was lauded by Labour opponents as well as Tory colleagues.

An indication, perhaps, of the "kinder politics" Mr Corbyn has said he wants to see in modern Britain, but a long way from what opponents say is the abusive nature of many Momentum supporters.

Seeing the wider picture and working collaboratively is, Mr Lansman says, a priority for Corbyn's Labour.

"You can't just take up the cause of one side, you have to take into account the interest of another. I think to some extent Jeremy always has.

"That is about moving from opposition to government. That doesn't mean he will have to change his principles, he is not someone who departs from those - he has integrity.

"Momentum has an important role to play in advocating and supporting the policy, and developing the movement, that Jeremy wants to see built."