The GMB’s Amanda Gearing said members walking out at the Coventry warehouse deserved “decent pay”.
She said: “When the workers have got nothing to lose, you see them coming out fighting. The depots are just horrendous places to work.”
Workers are under huge pressure to hit targets, she said, adding: “We’ve got injuries and more ambulances coming here than any other UK warehouse.”
Striker Garfield Hilton said even trips to the toilet are questioned, by managers who want “every minute maximised”.
An Amazon spokesman said: “The vast majority of ambulance call-outs are related to pre-existing conditions. You can use the toilet whenever you like.”