The investigations demonstrate that despite 953 license approvals, affecting 556,706 workers, a significant number of processors are continuing to use labour provided by illegal gangmasters.
Using unlicensed labour providers in the food processing and agriculture sectors has been a criminal offence in the UK since 1 December 2006, following the creation of the Gangmasters Licensing Authority (GLA).
Executives at companies using illegal labour or unlicenced gangmasters could face up to six months imprisonment, a £5,000 (€7,400) fine, or both.
Gangmasters are individuals or agencies who provide temporary labour to food manufacturers, among others.
A total of 39 cases, involving 31 firms and eight individuals, is being investigated and the 'vast majority' are food processors, a GLA spokesperson told FoodProductionDaily.com.
In particular a food processor operating in England is being considered for prosecution by the Department for Food and Rural Affairs (Defra), the spokeperson said.
Evidence gathered by the GLA against another alleging executives used an unlicensed gangmaster in Scotland is under review by the government prosecutors.
A further case involving a processor using unlicensed gangmasters in England is being reviewed by the GLA to determine whether a caution against the executives involved is the appropriate action, the spokesperson said.
A decision on whether the prosecutions will proceed is expected within two months.
The GLA declined to reveal the identity of the processors because the investigations are ongoing and no charges have yet been made.
A further 18 cases, in addition to the 39 already under investigation, primarily relate to the illegal use of unlicensed sub-contractors are also under consideration by the GLA.
The investigations will be initiated once sufficient evidence gathering is complete, the spokesperson said.
Of those cases being considered for further investigation, some are food processors.
In response, the Food and Drink Federation (FDF), an industry organisation, denied that members used unlicensed labour.
In a statement to FoodProductionDaily.com the organisation said members had experienced no problems in sourcing legal temporary labour since the legislation came into effect.
"We were very much involved in the legislation and the consultation.
As a result our members were fully aware," said an FDF spokesperson.
The GLA also said that 21 applications for gangmaster licences had been rejected since the legislation came into force.
Three of those rejected still have the right to appeal the decision.
While the majority of gangmasters used by the food processing sector have become licensed, those used by the fisheries industry have ignored the regulations, the GLA spokesperson said.
To date, there had been no license applications for gangmasters providing labour to the fisheries and shellfish collecting sector, which comes under regulation on 6 April 2007, said the GLA.
This is alarming given 23 Chinese cockle pickers died in Morecambe Bay in 2004, prompting both legislation and the GLA's creation.
The GLA spokesperson said he expects gangmasters in the shellfish gathering industry will continue to operate without a licence, although the regulator was still hoping that some will come forward and apply for a license before the deadline.
The GLA said plans were in place to deal with those that do not.
This information comes as a report published by Anti-Slavery International claimed that forced and illegally trafficked labour was being used by food processors and other industries within the UK.
The GLA said it was willing to work with any company or individual that wanted advice or assistance relating to the legislation, and urged anybody with information about unlicensed activity to contact 0845 602 5020.
Powers provided under the Gangmasters Licensing Act allow the regulator to investigate, gather information, interview, arrest and prosecute those operating in the sector without a licence.
The same powers apply to those using unlicensed labour providers.