Fast fashion firm Asos axed contracts with a string of clothing suppliers after it uncovered possible serious threats to workers' health, safety or human rights.
The company's inspectors found major ethical breaches at almost a quarter of suppliers visited in 16 countries, including seven sites in the UK, according to a 2018 report leaked to the Telegraph.
Since then, Asos has reduced the risk to 10pc.
In the UK it has ditched one supplier in Leicester - the city at the centre of allegations over poor working conditions following criticism of rival Boohoo - due to a lack of business, as well as ending deals with two other sites for ethical reasons.
The leaked document dates from May 2018 and summarises concerns about Asos's supply chain after it inspected sites to check on working conditions as part of an ethics drive. It covers data gathered over the previous 12 months.
Factories that were visited were divided into categories, the most serious being "red" and "red critical", where inspectors had found evidence of immediate, serious or long-term "risks to workers' safety, welfare or human rights" or significant gaps in management systems.
The company uncovered these problems in 185 sites around the world or nearly 25pc of the suppliers it inspected. This included seven factories in Britain which were ranked as "red", the second most serious category.
Asos said that since the report was completed, it has cut the proportion of high risk sites worldwide to 10pc after working with bosses to improve conditions and ditching companies that refused to change.
Only one is now the highest "red critical" level, a Vietnam operation, which faces bribery allegations and which Asos plans to stop using.
Two British factories are deemed as high risk, both of which are expected to receive a follow-up audit in August, and both of which have had visits or calls in the last week to work through unresolved issues. The remaining are either largely compliant or will soon be compliant with the retailers’ standards.
Only 8.5pc of the total sites were deemed "largely compliant" with regulations that met the company’s ethical standard in 2018.
The 2018 report also shows that at the time, the UK had the highest number of breaches in a category called "employment is freely chosen", with 10 cases reported.
This includes issues such as whether workers' passports were confiscated, if their movements were restricted and if they were forced to do overtime against their will.
Sources close to Asos say it carries out audits for all its main factories at least once a year and it has since dumped suppliers that fall short of its standards. It had also started inspecting secondary suppliers from last year.
The ethical breaches uncovered by Asos, which seeks to hold itself to high standards on the issue, will raise questions about how widespread malpractice is in the fashion industry and whether rivals conduct similar wide-ranging audits to spot failings.
An Asos spokesman said: "At the heart of our industry-leading ethical trade policy is a commitment to rigorously and regularly audit our supply chain to ensure workers’ rights are protected and respected.
"This internal report, which is more than two years out of date, shows how seriously we take this process, and since it was created we have made – and continue to make – significant improvements across our entire global supply base."