Not since 21 Chinese cockle pickers drowned in Morecambe Bay two years ago has the plight of vulnerable workers taken up so many column inches.

At the height of the British soft fruit season, as supermarkets pile up punnets of strawberries and raspberries for sale at half price, two big British fruit farms are under the spotlight for alleged mistreatment of their seasonal labourers. S&A Produce in Herefordshire has been accused by unions of enforcing 14-hour working days and charging for access to medical care - something the company vigorously refutes. Meanwhile, Mansfield Farm in Kent is under investigation by Marks & Spencer and Sainsbury following allegations that it has not been complying with minimum wage rules. Sainsbury suspended supplies from the grower last week.

The two cases highlight mounting concerns over the scope of new gangmasters' legislation, which comes into force on 1 October. The government hopes this new swathe of red tape will make it harder for the murky industry of labour provision to abuse seasonal migrants, but there are real fears over the extent of its take-up by the industry.

Many growers and packers recruit directly from abroad, taking them outside the scope of the legislation. Only those using labour from a provider not accredited by the Gangmasters Licensing Authority, and the providers themselves, will be liable for a hefty fine and up to 10 years in prison.

Neither of the farms being investigated admitted to using gangmasters, which means that, increasingly, it is the retailers that are having to police their own supply chains.

Each headline about poorly-treated migrant labourers picking strawberries for the supermarkets has an adverse impact on the £150m-a-year strawberry industry.

For example, Marks & Spencer conducted a lightning review of its other soft fruit suppliers, checking for compliance on pay and conditions. Meanwhile, Tesco has assured the public that all its suppliers are expected to meet strict, independently audited criteria for labour standards. And Sainsbury said it has informed growers and fruit marketers about new legislation, poised to restrict the use of gangmaster labour.

Jeremy Moon, head of the International Centre for CSR at Nottingham University, says it is a role that big supermarket players are getting used to. "Retailers have taken responsibility for the way products are produced and the sourcing of primary produce."

Needless to say, it doesn't come cheaply. S&A's HR manager Gilbert Savory says: "It costs us an awful lot of money and it costs the supermarkets a lot of money. It rubs off on everyone involved."

The irony, says NFU wages spokesman Bob Fiddaman, is that growers are being forced to sail close to the wind because of the price pressures from the supermarkets. "We agree minimum payment rates every year and expect the industry to follow that mandate. But will supermarkets recognise that my labour costs also rise 6% on 1 October when the minimum wage is increased?"

Worryingly, the two cases could be just the tip of the iceberg, according to The Transport and General Workers Union, which called a picket of S&A last week. It claims to have received complaints from workers at other UK farms.

With picking season in full swing, few would bet against

more claims of abuses against workers emerging before the summer is out.