Three men were arrested on suspicion of human trafficking for the purpose of labour exploitation and 60 migrant workers from eastern Europe, aged 15 to 67, were being interviewed as potential victims and witnesses.

They had been brought to the area by bus from North-amptonshire and were picking leeks for Emmett UK when the field was raided on Tuesday.

Police said that the Fosdyke-based firm is assisting officers but is not being investigated in connection with any criminal matter.

In a statement Emmett UK said: "Emmett UK and its suppliers take the welfare of their staff extremely seriously and we are assisting the police as much as we can with their inquiry.

"Our primary concern at this time is for the people who have been working for us. The provider in question was licensed in June 2007 and has been audited twice by us and once by the Gangmasters Licensing Authority, most recently in October 2008."

Two hundred staff from nine agencies were involved in Operation Ruby, which centered on allegations that people were recruited through advertisements and agencies in eastern Europe and had travelled to the UK on the promise of work.

When they arrived it is believed that their documents were taken from them and much of the wages they earned were withheld to pay for their housing and transport costs.

Det Supt Glyn Timmins, director of investigations at Northamptonshire Police, co-ordinated the operation and said it was the culmination of months of liaison, investigation and planning.

He said: "This has been a co-ordinated, large scale operation as part of an investigation into the activities of what we believe is an organised crime group that has exploited people as cheap labour in fields across this region.

"The fact that so many organisations have come together to act in this way is a demonstration of how seriously we take our responsibility to support people who have been exploited in this way and to disrupt this unacceptable activity."

East Midlands Foreign National Crime Team, North-amptonshire Police, the UK Human Trafficking Centre, UK Borders Agency, Serious Organised Crime Agency and GLA were involved in the operation.

Police also searched 21 houses in Kettering and the wider Midlands areas plus business premises in Market Harborough.

Four men and a woman were also arrested at Kettering and Wellingborough and taken into custody on suspicion of people trafficking and money laundering.

All those arrested are from Kettering and Coventry. One man was released without charge and the others have been released on bail pending further inquiries.