The Equality and Human Rights Commission has begun contacting up to 100 Labour figures it wants to hear from as part of its investigation into antisemitism, warning they may be committing a criminal offence if they withhold information.

Formal letters are being sent to the whistleblowers who appeared in Wednesday’s Panorama documentary, as well as some serving Labour staffers, and members of the party’s national executive committee (NEC), the Guardian understands.

Recipients are warned that failing to comply with requests for information could mean they have committed a criminal offence.

The EHRC launched its inquiry in May, saying at the time it would seek to “determine whether the Labour party has unlawfully discriminated against, harassed or victimised people because they are Jewish”.

Q&A What is the EHRC and what are its powers? Show The Equality and Human Rights Commission (EHRC) was formed in 2007 to promote and uphold equality and human rights laws across England, Scotland and Wales. It bought together the work of three previous bodies -the Commission for Racial Equality, the Disability Rights Commission and the Equal Opportunities Commission. The Commission's role includes enforcing equality legislation on 'protected characteristics' - age, disability, gender, marriage, pregnancy, race, religion and sexual orientation. The EHRC's powers range from simply providing advice to organisations, to holding inquiries to ensure that equality laws are enforced and to work towards ending discrimination and harassment. Ultimately the EHRC can take organisations to court in order to force them to comply.

It is only the second time the watchdog has launched a full statutory inquiry using the powers it was granted in the Equality Act of 2006 – the first being an investigation into the Metropolitan police.

Labour has insisted throughout that it will cooperate with the EHRC. The body does not have the power to fine individuals, but can make legally binding recommendations for changes in policy.

Its statutory powers allow it to demand that evidence, including electronic communications and documents, be handed over.

In giving evidence, former staffers served notice by the EHRC would no longer be bound by the non-disclosure agreements they signed when they left, while serving Labour employees would be protected under whistleblowing laws.

The latest development in the investigation, which could take up to a year to complete, came as the war of words over the party’s handling of antisemitism complaints continued to escalate.

Labour’s general secretary, Jennie Formby, hit back against Corbyn’s deputy, Tom Watson, accusing him of “traducing” her while she is undergoing treatment for breast cancer.

Watson wrote to Formby after the Panorama documentary, asking her to share with him the party’s submission to the EHRC investigation.

He also asked for reassurances that all relevant data would be retained – hinting he feared evidence could be destroyed. Separately, he complained publicly that Labour had “smeared” the staffers who featured in the documentary.

Before Panorama aired, a Labour spokesman claimed the participants were “disaffected” and had “axes to grind”.

Replying to Watson on Thursday evening, Formby said she was “very disappointed” he had criticised the party’s response on antisemitism and accused him of abusing his position.

“The party has at all levels consistently shown that it recognises the vital importance of combating antisemitism, yet you consistently abuse your considerable platform to denigrate any progress that has been made and any individual that is involved in that,” she said.

“Traducing my reputation and publicly attacking me when you know I am undergoing chemotherapy and am unable to respond in the media is another example of the inappropriate way in which you choose to discuss this issue.”

She also defended Labour’s treatment of the individuals featured in the documentary, some of whom said they had suffered mental health problems as a result of the issue.

Formby said she was “very concerned to hear for the first time the distress suggested by some of our former staff members”, adding, “to be clear we were not made aware of them at the time”.

The shadow chancellor, John McDonnell, a key ally of Corbyn, threw his weight behind Formby, tweeting it “goes beyond my understanding” that the letter was sent “when he knows she’s undergoing chemotherapy”.

He wrote: “I just don’t understand why the deputy leader of the Labour party uses the media to demand information from Labour’s general secretary @JennieGenSec, which has already been offered to him.”

Claudia Webbe, who sits on Labour’s ruling national executive committee, called for Watson to consider his position – a demand that was retweeted by allies of Corbyn, including the shadow home secretary, Diane Abbott.

@tom_watson your engaged in public attacks on the Labour Party’s General Secretary @JennieGenSec despite your clear knowledge she is receiving chemotherapy treatment for cancer. This is not behaviour befitting of the Office of Deputy Leader. You should consider your position? — Claudia Webbe (@ClaudiaWebbe) July 11, 2019

The fallout from the programme, based on interviews and leaked emails, continues to cause serious ructions in the party.

Corbyn’s team believe Watson is seeking to use the issue as a way of undermining the leadership, just as divisions on Brexit policy had been papered over after the shadow cabinet agreed to support remain in a future referendum.

Watson said a document produced for the EHRC’s investigation should be made public because “only sunlight can disinfect Labour of antisemitism now”, but Formby insisted she had offered to meet him and provide any information he demanded.

Formby took over last year as general secretary of the party, with the support of the leadership, and was tasked with speeding up the response to complaints of antisemitism.

During her treatment for cancer, Formby’s duties are covered by the party’s executive directors, who include Corbyn’s close aide Seumas Milne and chief of staff Karie Murphy.

Formby attended the shadow cabinet meeting earlier this week at which the new “settled position” on Brexit was agreed.