McDonald's employees were forced to sell adulterated food, serve undercooked burgers and work on kitchen floors covered with two inches of raw sewage, the High Court in London was told yesterday.
The accusations were made by Simon Gibney, a former manager at the burger giant's Colchester store.
Mr Gibney worked at the outlet, a former store of the year, and therefore a model for other British McDonald's restaurants to follow, from 1984 to 1987.
He was seen as a rising star by the company and by the age of 20 was promoted to the company's management training programme, a year earlier than was usual.
Mr Gibney made the accusations during a libel trial, now in its 16th month. The burger chain accuses two environmentalists from north London of distributing a leaflet which allegedly claims McDonald's sells food linked to heart disease and cancer, abuses its workforce and is responsible for acts of environmental destruction.
Helen Steel and Dave Morris, dubbed the " McLibel Two" by their followers, deny libel but claim the leaflet is a fair comment on the company's activities.
Mr Gibney said that on "at least two occasions" sewerage came flooding up from the floor vents in the kitchen. On one occasion the employees were forced to stand on bun trays to keep above the rising sewage.
The flooding, said Mr Gibney, resulted from the management's refusal to replace special equipment which allowed them to clean and re-use the fat. As a result, they were forced to pour large quantities of fat down the drains, which solidified and blocked them.
In a statement, Mr Gibney said that the area supervisor, Frank Stanton, ordered the store manager "to water down drinks, syrups, ketchup, mustard, milk shake mix and use less lettuce in the burgers".
To further boost productivity, staff were forced to keep food for more than the recommended 10 minutes. It was often held for 20 minutes or more. At other times, part-cooked burgers were served.
Staff under the age of 18 were also forced to work long hours and sometimes past midnight, contrary to the law at the time.
Workers also did not have their wage rates increased in line with Wage Council rules.