British pop band the Spice Girls said they will fund an investigation into a Bangladesh factory over its treatment of women who make T-shirts for a charity campaign backing equality.

Factory workers making the tops in Gazipur, Bangladesh, earned as little as 35 pence (45 cents) an hour, were forced to work up to 16 hours a day and suffered insults and harassment, according to an investigation by the Guardian newspaper.

The T-shirts, which were commissioned by the Spice Girls and retail for about 20 pounds, aim to raise money for a campaign by British charity Comic Relief championing gender justice.

The group topped charts in the 1990s with a "girl power" mantra that appealed strongly to young women, selling the idea that girls from ordinary backgrounds could make it big.

The slogan '#IWannaBeASpiceGirl' appears on the front of the tops, while the phrase 'gender justice' is printed on the back.

A spokeswoman for the Spice Girls - who announced in November that they would reunite for a tour of Britain this year - said they were "deeply shocked and appalled by the claims".

"The girls will personally fund an independent investigation into the working conditions of this factory," the spokeswoman said. "Equality and the movement of people power has always been at the heart of the band."

GIRL POWER?

Comic Relief said both the charity and the band had carried out ethical sourcing checks on the online retailer commissioned to make the T-shirts, Represent, but that it had subsequently changed manufacturer without their knowledge.

Represent could not be reached for comment, but Comic Relief said the retailer had taken full responsibility for its choice of supplier, and that it would refund customers on request.

Workers making the T-shirts told the Guardian they were forced to do overtime, made to work despite poor health, and verbally abused with insults such as "daughter of a prostitute".

Low wages and trade deals with Western countries have turned Bangladesh's garment sector into a $30 billion industry accounting for 80 percent of the country's exports.

Bangladesh has been hit by violent demonstrations in recent weeks after thousands of workers took to the streets demanding better pay. Garment owners agreed to raise wages last week but many workers rejected the pay hike and launched fresh protests.

From clothes and cosmetics to shrimp and smartphones, major brands face rising regulatory and consumer pressure to ensure their supply chains are free of labour abuses as the world looks to meet a United Nations goal of ending modern slavery by 2030.

Yet consumer trust seems to be at an all-time low, according to Joanna Ewart-James of the anti-slavery group Freedom United, who highlighted the irony of a online t-shirt intended to back gender equality being produced by women in a exploitative workplace.

"All buyers - charities as well as companies - must take contractual responsibility for ensuring decent working conditions in their supply chain," the executive director of the global campaign group told the Thomson Reuters Foundation.