The visits by 15 GLA officers were part of a four-day exercise, which took place earlier this month across Lincolnshire and Lancashire, where workers were harvesting and packing vegetables or preparing plants at nurseries.
In 16 inspections, the gangmasters interviewed were able to demonstrate that GLA requirements were being met, the authority said. But in four field visits to holdings in Lincolnshire, interviews by officers found that workers were allegedly paid below the legal minimum, without contracts or arrangements for holiday or sick pay.
Allegations also included workers being left in a field at 6am without supervision and transport to pick produce until they were collected at 4pm, and an untaxed van used as worker transport.
The workers interviewed were mainly of Indian, Polish and Lithuanian nationalities.
The exercise was part of Operation Ajax, which started in June. During an 18-month period, 20 to 30 surprise raids will target areas where the GLA believes workers are being exploited.