The shadow chancellor has expressed his concern that the row over antisemitism in Labour may have an effect on the election result, adding that the party had “done everything we can possibly do” to tackle the problem.

In a BBC One interview on Sunday morning, John McDonnell apologised to the Jewish community for “the suffering we’ve inflicted on them”. He added: “I say to them, we’re doing everything possible and we are going to learn more lessons and we want to be the shining example of anti-racism that the Labour party should be.”

Asked if he thought antisemitism in the party would be partly to blame if Labour lost Thursday’s general election, he said: “I worry that this has had its effect. We’ve done everything that I think we can possibly do. We’ve apologised to the Jewish community … We’ve always got to learn lessons, of course we have, all political parties. Because it isn’t just the Labour party.”

He added: “I hope … having gone through this horrible, horrible period, we come out of it now actually showing respect to the Jewish community and tackling this issue and enabling the Labour party to tackle antisemitism in our wider society.”

His comments came after a leak of documents from Labour’s disciplinary department to the Sunday Times, which included a recording of an official complaining that more than 130 cases were outstanding even though the majority were reported to the party 18 months ago and one had been on the books for more than three years.

The cases included members likening Jewish people to killer viruses, labelling them “bent nose manipulative liars” and calling for the “extermination of every Jew on the planet”.

It reportedly took 10 months for the party to expel a member in Nottingham who wrote that “Jews represent a viral infection that need to be completely eliminated” and said he wanted the “complete extinction of all Jews”.

Speaking to the BBC’s Andrew Marr Show on Sunday morning, McDonnell said it was his understanding that the number of outstanding cases was “well below” the 136 reported.

“It was a small number, but I don’t care how many it was,” he said. “One antisemite is too many, and what we’ve done is we’ve started kicking them out in numbers. The reports this morning, that I saw in the papers, cited a number of examples. Those people have been expelled or suspended. They’ve been done.”

McDonnell said he had previously made clear he was angry because the Labour party “weren’t quick enough or ruthless enough”. “The new procedures are dealing with that and that report today doesn’t take that into account,” he said.

Q&A What is ‘tactical voting’? Show Hide Under the first-past-the-post voting system, tactical voting is when you vote for a party that you would not normally support in order to stop another party from winning. For example, in a constituency where the result is usually tight between a party you dislike and a party you somewhat dislike, and the party you support usually comes a distant third and has no chance of winning, you might choose to lend your vote to the party you somewhat dislike. This avoids ‘“wasting” your vote on a party that cannot win the seat, and boosting the chances that the party you dislike most will lose.

The Equality and Human Rights Commission (EHRC) launched an official inquiry into antisemitism in the Labour party in May and is due to report next year. The final submissions on behalf of the Jewish Labour Movement (JLM) to the inquiry was leaked to the media on Friday, including 70 sworn testimonies from current and former staffers, and concluding that the party was institutionally antisemitic.

The issue of racism in the two main parties has dominated this election campaign, with Boris Johnson also coming under fire for racist and Islamophobic comments he and his party’s candidates have previously made.

The Guardian revealed on Friday that at least four ministers had gone on election campaigning trips to endorse Tory candidates who had, among other things, argued that Muslims had divided loyalties and blamed immigrants for bringing HIV to Britain.

The Conservative party also announced on Saturday that it was investigating three parliamentary candidates over antisemitism. One shared a video with an image implying that the billionaire George Soros, who is Jewish, controls the EU, while another asked whether a Jewish journalist was more loyal to Israel than to Britain.