Activists from the Unite trade union spelt out their message to Marks & Spencer shareholders using tomatoes as delegates arrived at the Royal Festival Hall for the clothing-and-food chain's annual general meeting on Wednesday.
Unite has been taking up complaints of alleged irregular labour conditions from workers who pick and pack tomatoes and other salad and fruits at Thanet Earth in Kent.
Thanet Earth claims to be the UK's largest greenhouse complex and boasts of its "exceptional green credentials".
"We want to highlight the rotten working conditions at Thanet Earth," says Unite regional industrial officer Dave Weeks.
"Most workers have no direct contract of employment and can be hired and fired at will. This is a major supplier to M&S which claims to be an ethical retailer. It is time for the supermarket to insist that Thanet Earth cleans up its act."
Thanet Earth this week issued a statement which said: "Having failed to satisfy Unite with our own internal investigation into their claims, we requested that the Gangmasters Licensing Authority conduct a full audit inquiry of our labour providers to provide a definitive response and to allow us to remedy any issues discovered.
"This inquiry was recently completed, but the GLA are as yet unable to publish a statement about the results. Without official confirmation from them, we are unable to discuss the matters in any detail but we maintain that we shall make information public at our earliest opportunity in order to clear the names of our labour providers and to protect our good reputation."
The statement continues: "We cannot move to a more permanent employment model and stay in business. Thanks to seasonal demand, weather forecasts and events such as sporting fixtures, our requirement for labour fluctuates to such an extent that it is impossible to predict how many people we need on site further than 24 hours in advance. To permanently employ more workers than we already do would be economic suicide. Every business in our industry works this way too."