The company behind H&M, COS and other clothing brands has been ranked highest in a study of how leading retailers impact conditions for garment workers.

But there was significant room to improve for others like The Just Group, which owns Just Jeans, Portmans and Peter Alexander, and Mosaic Brands, the company behind Noni B, Rivers and Katies.

Charity group Oxfam revealed the results in its Shopping for a Bargain report on Wednesday.

The report is an unprecedented look at how big brands’ business practices affect wages and conditions for the workers who make Australians’ clothes.

“The research reveals unfair purchasing practices are pressuring factories into adopting poor working conditions and paying unacceptably low wages,” Oxfam Australia’s chief executive Lyn Morgain said in a statement.

Those practices include aggressive price negotiation, inaccurate forecasting of orders, and last-minute changes to orders.

“It found that these poor purchasing practices of brands are making it impossible for factories to increase wages, despite many of the same brands making public commitments to ensure the payment of living wages,” she said.

“Instead, wages are trapping workers – mainly women – and their families in a cycle of poverty.”

A team of researchers from Oxfam, Monash University, and Bangladesh conducted more than 150 surveys and 22 interviews for the report.

They developed two sets of ratings for each retailer from the surveys — the rating it gave itself, and the rating factories in Bangladesh gave it.

The H&M Group scored 3 out of 4 based on factory feedback. Close behind were Big W, Kmart and Target Australia, with 2.5 out of 4.

The Just Group and Mosaic Brands scored 1.5.

Oxfam says H&M’s high ranking shows that higher prices don’t necessarily mean clothes have been produced sustainably and ethically.

Only half of the brands approached by Oxfam responded to the survey.

Of those that did, the biggest disparity between the way a brand scored itself and the way factories did was for Best & Less — which rated itself a 3.5, compared to the 2 given by factories.

All of the factories approached by Oxfam said that, despite brand claims to the contrary, brands will always terminate relationships with factories or switch factories if a factory cannot meet a lower price demand.

Seventy per cent of factories said that to deliver orders that exceeded company forecasts on time, they gave workers steep production targets and required them to work excessive overtime.

And 40 per cent agreed that they had accepted orders at prices below what it cost them to produce clothes safely and ethically.

Mosaic Brands head of compliance, Nic Williams, rejected the findings of the report.

“We are disappointed a report that appears to be based on limited or flawed data does not provide an accurate picture of how Australian retailers are operating in Bangladesh,” he said in a statement.

“Although Bangladesh is one of our smallest sourcing markets, the report does not state how many Mosaic suppliers were interviewed and does not reflect the extensive safety and wage auditing processes we have in place in Bangladesh and globally.”

The companies included in the report were Best & Less, Big W, Cotton On, H&M Group, Inditex (Zara), The Just Group, Kmart, Myer, Mosaic Brands and Target Australia. All make their clothes in Bangladesh.

Oxfam wants brands to stick to responsible purchasing policies and to negotiate in a way that accommodates labour costs.