Feminist campaign t-shirts could be withdrawn from sale by a women's rights charity over claims they were made in a sweatshop.
The Fawcett Society is investigating allegations in a newspaper that Mauritian workers were paid just 62p (€0.80) an hour.
In Britain, Labour's Ed Miliband and Harriet Harman have both been pictured wearing the tops.
Shadow Home Secretary Yvette Cooper says her party is already planning to put forward an amendment to the modern slavery bill next week:
The Mail on Sunday reported the shirts - which carry the slogan "This is what a feminist looks like" - were made at a factory in Mauritius where machinists earn around 80c per hour.
Whistles has released a statement saying the company is "shocked" by the claims and that an investigation will be carried out.
Our response to allegations made on the @fawcettsociety @ELLEUK t-shirt pic.twitter.com/ImN94YQxA3
— Whistles (@thisiswhistles) November 2, 2014
The shirts, which Whistles sells for £45 (approx €57) each, were produced in partnership with Elle magazine and the Fawcett Society, which promotes women's rights.
Politicians
Ed Miliband and Nick Clegg posed in the shirts for photographs taken by Elle magazine, and Labour's Harriet Harman wore one during Prime Minister's Questions this week.
Speaking to the newspaper, Fazal Ally Beegun, the president of the International Textile, Garment and Leather Workers Union, said workers at the Mauritius factory were poorly treated.
"The fact that politicians in England are making a statement using these sweatshop T-shirts is appalling," he added.
Dr Eva Neitzert, deputy chief executive at the Fawcett Society, said the charity had been assured by Whistles that the garments were produced to "ethical standards".
She said the charity was originally assured the garments would be produced ethically in the UK, and when they received samples in early October they noted they had in fact been made in Mauritius.
They were then assured by Whistles that the factory was "a fully audited, socially and ethical compliant factory" and decided to continue with the collaboration.
The Fawcett Society has said it would order the products to be withdrawn from sale if the reports are proven.
"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," she said.
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."
A Labour Party spokesman said: "This was a campaign run by Elle and the Fawcett Society to promote feminism and we were happy to support it.
"Anything else is a matter for Elle magazine and the Fawcett Society."
