Boohoo.com staff can get in trouble for smiling or checking the time, according to an investigation by Channel 4.
An episode of Dispatches looking at Britain’s cheap clothing industry suggests contracts are ended after “three strikes”, often for “minor” mistakes.
Other types of punishment include 15 minutes worth of pay being docked for being a minute late to a shift.
The retailer says the strike system is “not in line with current policies”.
The programme also appears to show examples of staff at a depot in Burnley being given two 30-minute breaks, one of which is unpaid, per 12-hour shift.
There are also claims some workers have to walk up to 25 miles during a shift.
In the show, a former Boohoo team leader says: “Guys check the time on their phones and we’ve been asked to give people strikes for that.
“In one instance, I received an email to ask me if I could give somebody a strike because somebody was smiling.”
After seeing some of the footage employment lawyer Liana Wood suggests: “If that individual is one minute late and they have to work 14 minutes unpaid, then that is potentially unauthorised deduction from wages.
“If that person is on the national minimum wage, that would bring their hourly rate below the national minimum wage… and if that is the case that would be unlawful.”
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Last year, MPs said they were investigating another online retailer, Asos, after claims the company was treating its workers “like machines”.
The committee told Newsbeat the investigation into Asos would be part of a general inquiry also looking at BHS and Sports Direct.
In response to the allegations raised in this programme, Boohoo says: “We are disappointed some individuals may still refer to ‘strikes’. This is not in line with our current policies.”
Boohoo says that all employees, including those with an agency, are paid at least the national minimum wage.
Dispatches – Undercover: Britain’s Cheap Clothes airs on 30 January at 8pm on Channel 4.