Lululemon, an athleisure brand whose £88 leggings are worn by celebrities and Instagram influencers, are sourcing clothing from a factory where Bangladeshi female factory workers claim they are beaten and physically assaulted.

The Canadian brand recently launched a partnership with the United Nations Foundation to reduce stress levels and promote the mental health of aid workers.

Yet young female workers at a factory in Bangladesh making clothing for the label gave detailed accounts of how they struggled to survive on meagre wages and faced physical violence and regular humiliation at the hands of their managers, who called them “whores” and “sluts”. The factory is owned and run by the Youngone Corporation, which supplies Lululemon.

Workers allege that:

Factory workers who break any rules or leave earlier than expected are verbally abused by management and hit. Some said they had been made to work despite ill-health

Some labourers are paid 9,100 taka a month (£85) – less than the price of one pair of their leggings, which sell for as much as £138.50. The sum is well below the 16,000 taka unions have been demanding and falls far short of living wage estimates

They are forced to work overtime to hit targets, saying they sometimes felt immense pressure not to leave their work stations

Lululemon said it has a strict code of practice, and does not tolerate any violations of this. The company said it would immediately launch an investigation. “There are currently no orders planned for this factory, and we will take appropriate action based upon the findings of our investigation,” a spokesperson said.

A UN Foundation spokesperson said: “We believe all workers should be treated fairly and welcome Lululemon’s investigation.”

One worker at the factory claimed she was slapped for leaving work early, after feeling unwell. “I was sick, so one day I left work at 5pm but I informed the line supervisor. He told his bosses I left without telling anyone and the next day, when I went to work, the technician in charge of my line slapped me,” she said.

She added: “He slapped me so hard my cheeks turned red and everyone asked me what happened. I couldn’t tell them the actual story. I just told them I had allergies.” The worker claimed she did not complain as she felt no one would care. She added that female workers were also called “prostitutes”.

“During last Ramadan, they created a new line and recruited new female workers. One day, a technician hit a label operator so hard on her chest. We could see she was in pain the whole day … She was lying in the back of the line for hours but our bosses did nothing about her.”

Lululemon launched a partnership with the United Nations Foundation (UNF) on 8 October, saying that for the last three years the leggings company and the UNF have been working together to develop a curriculum of evidence-based mindfulness, yoga and self-care training, and have conducted pilot testing to more than 500 aid workers in eight countries to date, including the United States.

The company has upped its sales and profit outlook and is now forecasting that it will haul in $3.8bn (£3.02bn) to $3.84bn in sales in 2019. Lululemon, which was started in 1998 by Canadian billionaire businessman Chip Wilson, as a purveyor of yoga pants, has been expanding and introducing new product lines.

But the clothes it produces are purchased from a range of developing countries where labour is cheaper, including Bangladesh, Sri Lanka, Cambodia and Vietnam, among other parts of the world.

The company’s website says: “We don’t own our manufacturing facilities so we take great care selecting our manufacturing partners. Our approach is to build relationships with factories that are aligned with our values and meet our sourcing expectations.”

It notes: “Our Vendor Code of Ethics applies to all countries where our products are manufactured, including Bangladesh, and ensures that our partners follow a single set of policies, regardless of legal and cultural differences.”

Lululemon was criticised in 2013 for taking months to sign the Bangladesh Safety Accord, after the factory disaster in Bangladesh that killed more than 1,130 people. A petition was launched for them to stand by their commitment to sustainability and ethics.

The Guardian spoke to one male worker at the factory, who claimed he had witnessed female workers being called “whore”, “prostitute” and “slut”.

He further alleged: “They treat men badly too. Sometimes they beat male workers too. I never got beaten myself but I have seen other people get beaten up.”

Workers also raised concerns about not being able to leave when they are sick, with one girl who had jaundice reportedly granted time off by the medical team at the factory but told by her production manager she had to keep working.

They claimed that they are sometimes understaffed, putting immense pressure on workers who needed to meet targets. “So workers have to work more. They can’t eat food or take rest properly which is very bad,” a labourer said.

Tuomo Poutiainen, country director for ILO Bangladesh: “Any type of violence and harassment at work is unacceptable … There is a need in the garment industry in Bangladesh to continue to raise awareness on sexual harassment in the workplace and establish active measures to address it.”

Anna Bryher, advocacy director for the campaign group Labour Behind the Label, said: “Women at the bottom of supply chains bear the brunt of fashion’s unrelenting push to be fast and cheap … It’s obscene.”

She added: “As outrageous as this story is, this isn’t a one-off. Women making our clothes in Bangladesh are routinely and systematically abused and harassed.”

An employee serves a customer at the Lululemon store in London. Photograph: Simon Dawson/Bloomberg/Getty Images

Bryher said that a recent survey found 80% of Bangladeshi workers making clothes from international companies said they had experienced or witnessed sexual harassment and abuse in the workplace.

A Lululemon company spokesperson said: “A responsible supply chain starts with us and we are committed to upholding ethical sourcing practices globally. Our Vendor Code of Ethics consists of best in class industry standards. We require that all vendors share our values and uphold a consistent set of policies that live up to our Code. We do not tolerate any violation of this Code. Upon learning of the reports about a facility in Bangladesh, we immediately launched an investigation. There are currently no orders planned for this factory, and we will take appropriate action based upon the findings of our investigation.”

Youngone Corporation said it is fully committed to providing a working environment in all its facilities that is safe, fair and just. It said employees are encouraged to share their opinion or launch a complaint through many different channels. The company said that when grievances have been found they have been addressed and led to disciplinary proceedings. An internal review has been launched.

• This article was amended on 16 October to clarify that Lululemon’s partnership is with the UN Foundation, not the UN, as a previous version stated.