Workers for the online retail giant Amazon are being threatened with the sack if they take four separate days off sick, even if they have a doctor’s note, an undercover Sunday Times investigation has found.

The retailer, which last year made more than £6bn of revenues in Britain, has a disciplinary system under which points are accrued for illness. Workers are issued a penalty point for each episode of sickness.

Workers are told that more than one point will result in a “series of counselling and disciplinary meetings” and between four and six points can result in dismissal.

Undercover footage: the pressures of working at Amazon

In one case, a woman who spent three days in hospital with a kidney infection was docked two points, reduced to one on appeal, despite providing a