Seventy Labour staffers past and present have given sworn evidence about antisemitism to the equality watchdog as part of a major investigation into the allegations dogging the party.

In a submission to the Equality and Human Rights Commission (EHRC), the Jewish Labour Movement (JLM) accused Jeremy Corbyn of creating a “welcoming refuge” for antisemitism in the party by “publicly supporting antisemites and antisemitic tropes”.

The leaked dossier also contains a bombshell allegation that staffers were ordered to upload cases onto USB sticks and deliver them to Mr Corbyn’s team to review – despite the Labour leader’s insistence that the complaints process was independent.

Mr Corbyn completely rejected any allegations of interference in the process and complaints he had allowed anti-Jewish hate to persist in the party.

His denial came after James Libson, a partner at the law firm Mishcon de Reya, which is representing JLM, said he had found evidence of “interference” in internal investigations into antisemitism cases.

“There are many, many outstanding complaints, many examples of interference and many examples of double standards in the way in which complaints are processed,” he told the Today programme.

Asked if there was evidence that the leader’s office was interfering in the investigations process, Mr Libson said: “There has been interference and that interference has unfortunately become institutional.

“Institutional in the sense that people affiliated with the leader’s office – and now in the actual unit that are investigating – and that at a more basic level, information is passing between the leader’s office and investigating unit.”

He added: “Passed by USB sticks, by WhatsApp groups, secret WhatsApp groups.”

The JLM, a Labour affiliate which represents 2,500 members, has asked the EHRC to rule on whether Mr Corbyn’s party is “institutionally antisemitic”.

Among scores of complaints it has compiled, one whistleblower said staff were told to deliver cases saved on USB sticks to the Labour leader’s office, and she was “instructed to lie to anyone who asked her where she was going, or what she was doing, when delivering the USB sticks”.

The 47-page document also contains allegations of verbal and online abuse suffered by party members, such as being called a “Tory Jew” and a “child killer” or told “Hitler was right” at constituency meetings.

It also detailed abuse of Jewish MPs, delays to handling of cases and questions about the personnel being allowed to handle cases.

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The document said: “There can be little doubt that the party is now suffering from this corrosive disease,” adding: “JLM hopes the commission will recognise the overwhelming evidence that antisemitic behaviour is rife within the party, and that the party is itself responsible for committing, permitting and encouraging antisemitic acts.

“The party’s structures, processes and employees have all contributed to an environment that is profoundly hostile to Jewish members and an organisation that is institutionally antisemitic.”

Mr Corbyn said he “completely” rejected the allegations when challenged on the campaign trail on Thursday, arguing that he introduced rules to deal with antisemitism when he took over as leader.​

He said: “We introduced an appeals procedure to deal with it and we introduced an education process so that party members understood the hurt that can be caused by antisemitic remarks or antisemitic behaviour.”

And he said that in July he introduced a process so “egregious” cases could be fast-tracked to an independent committee.

Mr Corbyn continued: “The total number of cases as a proportion of party membership is very, very low indeed – but one case of antisemitism is one too many.

“I do not interfere with cases and as I pointed out it’s an independent process.

“There are obviously some in-train. There always are a small number, but I’ve insisted that they deal with them as quickly as possible and as expeditiously and fairly as possible.”

Labour said it was committed to rooting out antisemitism and it is fully cooperating with the EHRC.

A party spokesperson said: “There are not 70 staff members who have worked on our disciplinary procedures in recent years. Any former staff would not know how our procedures currently work, after significant reforms, and former MPs’ staff or others who have not worked on disciplinary procedures would not have knowledge of those procedures or the numbers of cases.

“This document includes baseless assumptions about current staff’s workload and the untrue allegations about outstanding cases.

“The Labour Party is not institutionally antisemitic and complaints relate to a small minority of our members.

“We are the only political party that has published figures on cases of antisemitism, and we regularly account for the work we are doing to tackle it.