JD Sports and ASOS warehouses risk becoming like "dark satanic mills" after figures showed ambulance call-outs to their sites have risen steeply.
JD Sports saw 40 call-outs to its Rochdale site last year, while ASOS's unit in Barnsley was visited 45 times – a rate of almost one every week.
In the past three years, ambulances have been dispatched to JD Sports's Greater Manchester premises 117 times, and 148 times to the ASOS warehouse in South Yorkshire.
Matt Draper, of the Unite union, said: "The warehouses of some companies risk becoming the dark satanic mills of the 21st century.
"It doesn't have to be this way though. Where employers work with trade unions and treat people with respect there are fewer accidents and a better health and safety record."
The statistics do not reveal why the ambulances were called out, but they are likely to reignite concerns over working conditions at Britain's warehouses.
While the most high-profile scandal in recent years involved Mike Ashley's Sports Direct, whose site in Derbyshire was described as a "Victorian workhouse" by unions, JD Sports has also come in for fierce criticism.
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The chain was forced to launch an investigation in 2016 into conditions at its main warehouse in Rochdale after an undercover film highlighted issues with working conditions and pay.
ASOS also came in for heavy criticism from the GMB union over the data compiled by the Press Association.
Regional secretary Neil Derrick said: "ASOS bosses appear to be in denial about the inhuman conditions people have to work under in their Grimethorpe (Barnsley) warehouse.
"Their desperate drive for profit has created a damaging, anxiety-ridden workplace that is hazardous to our members' health."
However, the figures also show that Sports Direct's ambulance call-out rate has improved dramatically since 2016.
In 2018, just nine ambulances were called to the Derbyshire warehouse, compared with 17 in 2016, 18 in 2017 and more than 70 in 2013 and 2014.
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