A women’s rights charity behind a T-shirt campaign is investigating claims by a newspaper that the products were made in sweatshop conditions.

The Fawcett Society has said it will have the clothes withdrawn from sale if the reports are proved true. The Mail on Sunday claim the T-shirts, worn by politicians – such as Ed Miliband, Nick Clegg and Harriet Harman – proclaiming their feminist credentials, are made by female workers being paid just 62p an hour.

The paper said its investigation found the shirts with the slogan “This is what a feminist looks like” were being produced on a factory in Mauritius where female machinists sleep 16 to a room.

The fashion retailer Whistles, which sells the garments for £45 each, described the allegations as extremely serious and said it would be mounting an urgent investigation.

The T-shirts hit the headlines last week when the Labour and Lib Dem leaders posed wearing them for photographs for Elle magazine as part of a campaign by the Fawcett Society – which receives all the profits – to promote women’s rights.

After David Cameron refused repeated requests to join them, Harman, the Labour deputy leader, wore one during prime minister’s questions.

The Mail on Sunday, which said it had toured one of six factories on Mauritius owned by Compagnie Mauricienne de Textile (CMT), which produces the garments, claimed that the workers were earning 6,000 rupees a month – equivalent to £120.

The paper said the figure was a quarter of the country’s average monthly wage and around half what a waiter earns.

Fayzal Ally Beegun, president of the International Textile, Garment and Leather Workers Union, told the paper: “The workers in this factory are treated very poorly and the fact that politicians in England are making a statement using these sweatshop T-shirts is appalling.”

Dr Eva Neitzert, deputy chief executive at the Fawcett Society, said they had been assured by Whistles that the T-shirts were produced to ethical standards.

She said the charity had originally been assured the garments would be produced ethically in the UK, and when they received samples in early October they noted they had been made in Mauritius.

The Fawcett Society told the Mail it was assured by Whistles that the factory was “a fully audited, socially and ethical compliant factory” and decided to continue with the collaboration.

“We have been very disappointed to hear the allegations that conditions in the Mauritius factory may not adhere to the ethical standards that we, as the Fawcett Society, would require of any product that bears our name,” Neitzert said.

“At this stage we require evidence to back up the claims being made by a journalist at the Mail on Sunday. However, as a charity that campaigns on issues of women’s economic equality, we take these allegations extremely seriously and will do our utmost to investigate them.

“If any concrete and verifiable evidence of mistreatment of the garment producers emerges, we will require Whistles to withdraw the range with immediate effect and donate part of the profits to an ethical trading campaigning body.

“While we wish to apologise to all those concerned who may have experienced adverse conditions, we remain confident that we took every practicable and reasonable step to ensure that the range would be ethically produced and await a fuller understanding of the circumstances under which the garments were produced.”

A spokesman for Whistles told the Mail: “We place a high priority on environmental, social and ethical issues. The allegations regarding the production of T-shirts in the CMT factory in Mauritius are extremely serious and we are investigating them as a matter of urgency.

“CMT has Oekotex accreditation [an independent certificate for the supply chain], which fully conforms to the highest standards in quality and environmental policy, while having world-class policies for sustainable development, social, ethical and environmental compliance.

“We carry out regular audits of our suppliers in line with our high corporate social responsibility standards.”

A spokesman for the deputy prime minister said: “Nick Clegg had no idea where these T-shirts were being made and can only assume that the Fawcett Society were unaware of the origins or they would not have asked him to wear it.

“He remains entirely supportive of efforts to ensure all women are treated as equals in this country and the world over.”

A Labour spokesman said the party was happy to support a campaign promoting equality, but added: “Anything else is a matter for Elle magazine and the Fawcett Society.”