They have been taken to safety in an undisclosed location in Kettering, where they are being asked about how they became virtual prisoners in this country, working on farms for little or no pay.

The state of their health and how long they were working in fields in South Lincolnshire where the rescue operation took place is also to be discovered.

Director of investigations at Northamptonshire Police Glyn Timmins said the victims were being interviewed about their ordeal. He said: "All the workers seem relieved and pleased with the police intervention.

"We are talking to them to firmly establish what happened."

Around 200 members of staff from agencies such as Northamptonshire Police, The UK Borders Agency and the Gangmasters Licensing Authority worked together for six months on the investigation, Operation Ruby.

They believe they have smashed a crime ring dealing with labour exploitation and are questioning eight people in connection with human trafficking and money laundering.

Intelligence about activity in Kettering originally arose as part of an East Midlands-wide initiative between police and the UK Border Agency.

As well as 12 properties in Kettering, police searched another nine in areas including Market Harborough, Peterborough and Coventry.

Experts say yesterday's crackdown is only the start in tackling what has been dubbed an "appalling" trade.

UK Border Agency regional deputy director Simon Excell said: "Both the UKBA and the police are determined to stamp it out and catch the ringleaders.

"We are taking action against more organised criminals, facilitators and employers who break the law."

He also said the UKBA would be taking tough action against employers who hire illegal immigrants, naming and shaming them, imposing heavy fines and even imprisoning them.

Deputy director of the Serious Organised Crime Agency (SOCA) Andy Baker told reporters more work is to be done to smash the trade.

He said: "Usually the people who are trafficked are here legitimately.
They have paid their passage and are subjected to a type of debt bondage, where they are made to work to pay their debt off. Some are kept like prisoners and some are subjected to violence. This operation is not the end of this. This is the start."

Area commander for Kettering, Corby, Wellingborough and East Northants, Chief Superintendent Paul Fell said: "The people who have been dealt with under this investigation are local people who have been committing serious criminal offences that bring untold misery to its victims.

"Local people shouldn't be concerned about what could be scare-mongering around hordes of illegal immigrants being in their community.

"These people had a right to be here but they were being abused."

Chief executive of the Gangmasters Licensing Authority Ian Livsey added: "I am sure there are other illegal gangs operating in the area."

"We want to hear from the public, who may be able to provide information about human trafficking."

Anyone who can help, is asked to call the GLA on 0845 602 5020 in confidence.