A Bangladeshi factory that produces clothes for Tesco, Sainsbury’s and Mothercare was forced to compensate an “outspoken” female worker after she was beaten up on the orders of management and threatened with being murdered, the Guardian has learned.

The woman claimed to have been “severely beaten up” by security guards and the HR and compliance management at the factory, which is used by the brand Stanley/Stella. She said she was robbed of her severance pay and told that if she protested she would be “killed and her body put in a cardboard box”, an industry watchdog report that endorses her account states.

On Sunday the Guardian revealed that at a separate factory used by Stanley/Stella, Spice Girls T-shirts designed to raise money for Comic Relief’s “gender justice” campaign were made by female workers paid just 35p an hour.

The report in which the incident is detailed does not name the factory, but Stanley/Stella confirmed that it occurred at Dird Composite Textiles, its largest supplier. The factory was also used by M&S until October last year.

The worker, who sat on the factory’s anti-harassment committee, was among 20 employees sacked last year after being accused of attacking factory management in an earlier incident, it is claimed.

The revelations are detailed in a report by the Fair Wear Foundation (FWF), a membership organisation paid by brands to “verify and improve workplace conditions for garment workers”.

The factory initially denied the allegations but later sacked the HR manager and paid the woman 68,250Tk (£630) in compensation after pressure from FWF.

In response to the revelations about the other factory producing their charity T-shirts, the Spice Girls said they were “deeply shocked and appalled” and would personally fund an investigation into the factory’s working conditions.

Comic Relief said it was “shocked and concerned”. Both said they had checked the ethical sourcing credentials of the online retailer commissioned by the Spice Girls to make the T-shirts, but it had subsequently changed manufacturer without their knowledge.

Dominique Muller, the policy director at Labour Behind the Label, said: “Only through real representative unions can these vulnerable workers have any power against big brands and suppliers. Stanley/Stella have been given a ‘leader’ status by the FWF despite this complaint and recent evidence which shows how far they are from being an actual leader.”

According to Fair Wear Foundation’s report, the group of 20 workers were rounded up and locked in a room on the factory’s seventh floor with management and local police, including the chief of the force.

Ordered to hand in their mobile phones, the group were told management had proof they were involved in the earlier incident, in June 2018, in which it was claimed they covered their faces with fabric and attacked several senior staff members and one visiting buyer.

The workers were told to sign resignation letters, take severance pay and “never come back”.

“They threatened them that if they would not accept, the police would arrest them,” the report states. “One by one, workers signed, after which they were escorted out of the building by three security guards.”

The woman wanted to sign first but was told to wait until the other workers had gone. After signing the resignation letter, the report states, the factory’s compliance and HR manager ordered security guards to take her to a different floor with no cameras and “beat her up”.

The report adds: “The security guards were also told that if she protested she should be killed and her body be put in a cardboard box. She was severely beaten up. The compliance and HR manager actively participated in the beating. Photographic evidence was provided that illustrate the complainant’s injuries and bruises.”

The severance money was also stolen during the attack, she said.

The woman subsequently called the Fair Wear Foundation, which advised her to go to hospital where she got a certificate from a doctor. The woman said she was reluctant to go to the police given the force’s involvement.

After denying the allegations, the factory eventually “reconsidered because of the potential risks of the complaint to … [its] business and reputation”.

The factory provided a resignation letter of the HR manager. The woman was paid her “legal dues” and was said to be “satisfied”.

Bruno Van Sieleghem, Stanley/Stella’s sustainability manager, said: “I understand you want to stress that some violence happens in factories producing for Stanley/Stella... This is a fact. It happened (we would have preferred not).

“Another version of the same fact is the way we reacted and solved the issue, fairly and respectfully. Together with factory top management and FWF representatives, after deep investigation, we got the full transparency about the real facts and took the only possible solution: fire the HR manager and give the legal compensation to the victim.”

Nabeel Ud Daulah, Dird Group’s managing director, said: “Dird Composite Textiles Ltd (DCTL) operates with the highest regard for ethical and moral standards. In 2009 we were awarded best worker-friendly factory in Bangladesh.”

He said the company has established an anti-harassment system emulated across Bangladesh and that it had a zero tolerance policy on physical and verbal abuse.

A Fair Wear Foundation spokeswoman said: “The worker who filed the complaint didn’t want to go to the police. We did discuss this at length with her, but she was very explicit about it. She didn’t trust the police, mainly due to their earlier involvement in the incident.

“Both FWF and the factory investigated the case after it was filed. The factory denied that there was an actual problem. It was not until FWF put pressure on the factory [that] they decided to pay the legal dues to the complainant and fire the HR/compliance manager.”

A Sainsbury’s spokeswoman said: “All our suppliers have to meet our code of conduct for ethical trade and are regularly required to show compliance through independent, third-party ethical audits and on-site visits from Sainsbury’s colleagues. We are extremely concerned by these allegations and urgently investigating.”

An M&S spokeswoman said: “We were aware of the situation in June and closely followed the reports of the police, the factory and Better Work. Our last shipment with Dird was in October 2018 and we are no longer working this this supplier.”

Tesco said it was not aware of the incident and was investigating. Mothercare said it took staff welfare very seriously, worked “in close dialogue” with all factories and would be “reviewing the information” shared by the Guardian. It pointed to its code of practice and said it required an ethical audit and subsequent annual review.