As a result, the GLA has announced a new consultation to combat the mistreatment of such workers with the help of UK supermarkets.
The GLA is a government agency set up to protect workers from exploitation in agriculture, horticulture, shellfish gathering and food processing and packaging.
Mr Whitehouse said between 85 per cent and 90 per cent of workers are not exploited, but the remainder is still a "sizable number".
"That exploitation could be very nasty indeed. It may be quite low level like not paying holiday pay for example, but the really nasty end could be forced labour," he added.
The drowning of 21 cockle pickers at Morecombe Bay in 2004 was the most high profile case of the exploitation of food workers.
