By Martin Shankleman
BBC employment correspondent
The migrant workers were picking leeks in the field when officers arrived
Migrant workers taken into care following a human trafficking raid in Lincolnshire were harvesting vegetables for a supplier to Tesco and Waitrose, it has emerged.
They had been employed by a firm called A14, which supplied labour to Emmett UK, a private Spalding-based company.
Sixty migrant workers were taken from a field near Holbeach to a reception centre in Northamptonshire, where they are being treated as victims of crime.
Emmett said that it was not implicated in the police investigation.
Emmett UK said the workers had been "harvesting leeks" for the company and claimed that it had twice audited A14. Emmett added that the company had also been audited by the Gangmaster's Licensing Authority in October 2008.
But the GLA revealed that it revoked the license for A14 in August this year, and the company was only allowed to trade whilst the appeal was pending. The GLA is now considering whether to withdraw the the license with immediate effect.
Emmett UK's website says it supplies Tesco and Waitrose with vegetables.
Tesco said it was "extremely concerned" and pledged to support the authorities' investigation.
"We require the highest labour standards to be met through the supply chain," said a spokesman.
"We work closely with our suppliers and the Gangmaster's Licensing Authority to ensure labour providers meet the requirements of GLA," he added.
Operation Ruby, as it has been named, took six months to plan and involved officials from nine agencies, including Northamptonshire Police, the Serious Organised Crime Agency, and the UK Human Trafficking Centre.
It was alleged the workers, who had been recruited in eastern Europe, were trapped by the gang who placed them in squalid accommodation and forced them to work for up to 16 hours a day picking vegetables.
The UK Border Agency Regional Director, Simon Excell, branded the gang's alleged actions a "modern form of slavery".
"Human trafficking of any kind, whether for sexual or labour exploitation, is an appalling crime where people are treated as commodities and traded for profit," said Mr Excell.