Tesco, Sainsbury's and Asda have launched an investigation into the labour provided to one of its major vegetable suppliers, Bomfords.

It follows a swoop by the Gangmaster Licensing Authority, which resulted in seven labour providers to Midlands-based Bomfords losing their licence for infringements, including paying workers below the minimum wage.

Tesco said it was urgently investigating the allegations. "We expect our suppliers to meet strict criteria for labour standards," said a spokeswoman.

A spokesman for the GLA said one gangmaster in particular, Dynamic Workforce, had "intimidated some of its workers and mentally abused one".

GLA chief executive Paul Whitehouse added: "I'm dismayed Bomfords allowed this to go on because some of the failures by Dynamic would have been clear to them."

He accused Bomfords of paying at least one of the labour providers a sum that meant its workers could not have been getting the minimum wage. The GLA offers guidance on the amount companies should expect to pay a gangmaster for a given number of workers. Whitehouse said Bomfords was paying "signifcantly below this amount".

In the meantime, Bomfords has stopped using all seven of the providers, but it said in a statement it was continuing to use the individual workers. The company insisted it had followed the rules.

Bomfords employs 850 staff at its three packhouses in the Vale of Evesham. It is a major supplier to Tesco, which buys 15 million bunches of salad onions, 100 tonnes of asparagus and fresh peas and beans from the packer every year.

Employment law partner at Eversheds, Naeema Choudry, said: "There are probably more cases out there. People are going to start doing more due diligence on their labour."