The vice-chair of the Labour Movement has claimed it would be “folly” to think cases of anti-Semitism in Labour had not played a role in his narrow defeat in Hendon.

Mike Katz slashed Tory incumbent Matthew Offord’s majority from 3,700 to 1,000 as Labour enjoyed huge swings in its favour across London. JLM chair Jeremy Newmark also significantly narrowed the deficit to his Tory rival in Finchley and Golders Green, the seat with the largest Jewish community in the country. Both campaigns focused heavily on the prime minister’s approach to Brexit.

Katz said he had “fought to win” what was always going to a “tough challenge” and insisted “slashing the majority by two thirds was a huge achievement” against an incumbent in place since 2010.

While there were several potential factors in the narrow loss, he said: “It would be folly to ignore it [anti-Semitism] as a factor. I had many conversations with people who said ‘I like you, I like Labour’s policies, I’ve no love for the incumbent but can’t vote for your your party and its leadership because of recent events. It was a conversation that went beyond the Jewish community.

“It would be equally foolhardy to say that no Jew voted for me or Jeremy. I know many Jews were happy to do so. The fight against anti-Semitism continues. We want to get back to a situation where it’s not factor and if it is, it’s a factor in our favour.” Saying JLM would “redouble its efforst to fight for what’s right”, he added: “We’ve got to carry on engaging and ensure they do right and ensure they understand the deep hurt some comments have caused.”

After a night that also saw long-time JLM member Alex Sobel for the first time and supporter Rhea Wolfson dramatically reduce a heavy SNP majority in Livingston, Katz suggested British Jews “should take succour that it’s been a great night for members and supporters of JLM and we can assured the community’s interests will be safeguarded by them”.

Katz and Newmark were applauded for standing by some but faced heavy criticism from others for putting their names forward at this time, particularly against key friends of the community in Parliament.

“Some language used was motivated by partisan concerns as much as by community ones. I’m not looking for sympathy but we ask that people take we’re doing on good faith. One of the reasons I went for it was to try to rejuvenate the relationship with the party. I hadn’t stood in a heavily Jewish area people would say I’ve run from it.

“If the ‘why are we standing’ pieces were proven, we would have seen majorities increased to five figures. Instead they were cut to the bone.” The party’s manifesto, he insisted, was not “1983 radical” and “had a lot that people could get behind”.

He said he’d enjoyed meeting people throughout the campaign but it was “too early” to say whether he’d run again with the focus now on the bigger picture of the country’s future. But he said: “I feel there’s unfinished business in Hendon.”

None of the Barnet seats saw resource pumped in from beyond the borough. But Katz said the shock results in all three Barnet seats would knock “the complacent approach to all communities in the borough by the Tories. It will serve as a wake-up call on hard Brexit and attitudes towards public service cuts”.