Supermarket suppliers can expect even tighter scrutiny of their labour recruitment practices after the Gangmasters' Licensing Authority said it would be carrying out unannounced raids across the UK.

The GLA launched Operation Ajax this week, under which it will be undertaking 20 to 30 raids.

The raids will target businesses where worker abuses are thought to be prevalent, and will take place over the coming 18 months. In particular, they will focus on parts of the country with high gangmaster activity and where information has been received that businesses may not be complying with the law, said chairman Paul Whitehouse.

Operation Ajax is a further indication of the GLA's determination to clamp down on unscrupulous employers, and follows the announcement earlier this year that it would be working closely with retailers to prevent abuses within the supply chain.

"We have put together Operation Ajax to root out the rogues and catch the crooks," said Whitehouse. "In short, if you are abusing your workers, watch out, we are coming for you."

Sainsbury's said this week it would be looking to work closely with the GLA on approaches for targeting labour abuses, and planned to feed these into its own supplier audits.

There may be more people being mistreated in the horticulture and agriculture industries than many realise, according to Owen Warnock, food sector and HR specialist at international law firm Eversheds. "There are significant numbers of people employed by gangmasters and whose rights are not being respected," he said.

Warnock advised suppliers to implement their own system of checks.

Last week the first person to be convicted under the Gangmasters (Licensing) Act, Fiona Jane Clark of Perth, was sentenced to 18 months' probation and 140 hours of community service for operating without a licence.