Working conditions at Britain's largest greenhouse complex are being investigated by the Gangmasters Licensing Authority (GLA) after claims that agency workers at the vegetable growing site in Kent are being treated like "sweatshop labour".

Protests organised by the Unite trade union were held on Thursday outside Broadstairs branches of Tesco and Marks and Spencer, which are among the supermarket chains supplied with vegetables from the 220-acre Thanet Earth.

Unite has submitted allegations to the GLA, the body responsible for regulating agency practices, including claims that workers have been systematically denied holiday pay, denied work as a result of taking holidays, and been employed for years on a casualised basis without fixed contracts.

Thanet Earth grows cucumbers, peppers and tomatoes that are supplied to Tesco, Sainsbury's, Asda and Marks and Spencer. It has been employing more than 250 workers in recent weeks to meet demand.

The facility emphasises green credentials. It is a joint venture owned by the Fresca Group, the country's largest privately owned fresh produce supplier, and three specialist grower companies.

In a statement Thanet Earth insisted that the agencies supplying its labour were fully audited and licensed by the GLA. All legal safeguards were in place and an internal investigation carried out after allegations were raised by Unite in March 2010 was concluded satisfactorily.

It said it had invited the GLA to carry out an investigation after Unite, which it accused of having "a fundamental opposition to the use of agency workers", raised allegations again in March of this year.

"We strive to make Thanet Earth an attractive, welcoming place to work and we care about the treatment of all workers on site – be they employed by an agency or by one of the Thanet Earth businesses directly."

Unite claims that the vast majority of the workforce have no permanent contract, no job security and work through four different agencies in a system of permanent casualisation.

A Unite regional officer, Dave Weeks, said: "We're taking our fight to the top of the food chain – those supermarkets that employ Thanet Earth. They have an obligation to make sure workers who help generate their healthy profits are treated fairly from root to basket. We know that many shoppers think if you buy local you buy ethical – that's the message Thanet Earth might have you believe – but these workers have being treated like sweatshop labour."

An unnamed worker said: "We are treated like sweatshop labour. The agencies have done whatever they want, and Thanet Earth and the supermarkets have let them. When we have asked for our rights we have been told: 'You can find another job.'

"We are disposable people to Tesco, Sainsbury's, M&S and the rest, despite them agreeing to respect workers' rights."

Neil Court of the GLA told the BBC: "We believe that Thanet Earth Marketing are a responsible company and I don't think people should have any concerns on the basis of the allegations that have been made.

"If we find evidence of non-compliance we will take action against the relevant licensed labour providers ... even if that means removing the licence of that labour provider."

The GLA is expected to announce its findings in May.