Charity “girl power” T-shirts sold in the UK are made at a Bangladeshi factory where more than 100 impoverished workers claim to have been sacked after striking in protest at low wages, it can be revealed.

The £28 garments are sold online by F=, which claims to be “all about inspiring and empowering girls”, with £10 from each T-shirt donated to Worldreader, a charity that supplies digital books to poverty-stricken children in Africa. Television presenter Holly Willoughby recently reposted a 2017 picture of her and Spice Girl Emma Bunton wearing the T-shirts.

The Guardian has established that the garments were made by Bangladeshi firm Dird Composite Textiles, where some workers earn as little as 42p an hour and complain of harassment. In one case, a female employee was beaten on the orders of the management and threatened with murder.

After being contacted by the Guardian, F= stopped selling the T-shirts and Worldreader pledged to cease accepting donations “until the situation is resolved”.

Machinists at the factory say they have been sacked en masse after striking over wages in January. They are among more than 7,500 employees at 27 factories in Bangladesh who have lost their jobs in recent weeks, according to union leaders, amid widespread protests and strike action over the imposition of a new minimum wage – which critics argue is too low – for the country’s garment industry.

The new minimum wage for the sector is 8,000 taka (£71.34) a month, half what the 16,000 campaigners had been demanding and well short of living wage estimates. Meanwhile, some higher-grade factory workers who already earned more than 8,000 taka a month received only small increases, it is claimed.

Kalpona Akter, executive director of the Bangladesh Centre for Worker Solidarity, who worked as a child labourer in textile factories, said: “The huge number of dismissals over wage protests shows how workers’ voices have been suppressed and how they are lacking freedom of expression.

“The workers that got fired know the law and their rights. In many cases they were union leaders in their respective factories. These workers are picked intentionally so there is no voice left in a factory to fight against retaliation and form a union.”

Dird insists that the workers who left resigned of their own volition.

The Fair Wear Foundation (FWF), a membership organisation paid by brands to improve working conditions, is investigating after a complaint was received about the workers’ plight at Dird’s factory.

The news comes after the Guardian revealed last month that an “outspoken” worker at the same factory claimed she was beaten up on the orders of management and threatened with murder.

The woman, who sat on the factory’s anti-harassment committee, 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”, according to an FWF report.

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

A separate Guardian investigation last month revealed how Spice Girls T-shirts sold to raise money for Comic Relief were made by Bangladeshi women at a different factory earning 35p an hour, who claimed they suffered harassment and were forced to work up to 16 hours a day.

After Willoughby reposted the picture of her and Bunton wearing the “girl power” T-shirts recently, the Twitter feed of F= shared the picture and said the items were in “huge demand”.

Representatives for Willoughby and Bunton declined to comment, but it is understood neither was paid to wear the garments or post them on social media.

F=’s website claims that they are “made in a Fair Wear Foundation certified factory, which means it is vetted for good working conditions and fair wages and by using organic cotton we drastically reduce the use of water”.

The shirts are made by Stanley/Stella, the same Belgian brand that made the Spice Girls T-shirts. The Guardian has established, via a code printed on the label, that the garments were made at Dird Composite’s factory in Bangladesh.

An FWF spokeswoman said: “Over 100 workers claim to have lost their jobs. We did indeed meet with the factory and Stanley/Stella. There were some discrepancies between what we heard from the factory management and the complainants’ stories and their personal files.

“The factory is in the process of paying legal entitlements, such as due salaries, provident fund and severance pay to all the workers concerned. FWF and Stanley/Stella will keep a close eye on this.”

Danielle Newnham, who founded F= with her sister, said: “We have asked Stanley/Stella about these reported resignations – they have explained that this only happened recently and is being investigated by the Fair Wear Foundation and Stanley/Stella’s country manager in Dhaka.

“To clarify, we print our T-shirts here in the UK, but we are always concerned if anyone is treated badly – our entire mission is based on empowerment and if we receive evidence of poor treatment, we would look for another supplier immediately.

“All the research we have done in the past showed Stanley/Stella to be one of the best manufacturers both in terms of sustainability and working conditions they urge their suppliers to uphold, hence we used them. However, we are wholly dedicated to empowerment and have therefore closed our entire shop whilst any issues are being looked into.”

A Worldreader spokesman said: “Worldreader was saddened to hear about the conditions in the factory where these shirts were produced. Prior to accepting donations from the sales of ‘Girl Power’ T-shirts, we entered into a contract that ensured the shirts were ‘Fair Wear certified’. Worldreader has agreed with F= that we will cease accepting donations from the sale of these shirts until the situation is resolved.”

A Stanley/Stella spokesman said: “In any country facing fast economic development, some progress is still to be made with regards to social compliance. Wages are still too low, even after a 40%-50% increase last December, and overtime is often needed to support the normal living wage. Unfortunately, some gender discrimination can also be found. However, by producing garments in Bangladesh, responsible European brands – like Stanley/Stella – can drive a positive change.”

Dird’s group managing director, Nabeel Ud Daulah, said that the company operated “with the highest regard for ethical and moral standards” and denied any suppression or targeting of worker representatives. He said that the staff who left resigned “because they were not satisfied with the new wage structure that was announced by the Bangladesh government” and that “all workers that resigned have either received their due financial entitlements or have been contacted to collect their due entitlements”.

Pointing to benefits provided to workers, including scholarships for their children, insurances and bonuses, he added: “We take such allegations extremely seriously and have provided full disclosure and cooperation to Fair Wear Foundation.”