Last week's recall of fresh basil from leading supermarket chains was prompted by a study that has uncovered further damning evidence of a possible link with the rise in human cases of salmonella poisoning, The Grocer can reveal.

The Health Protection Agency launched the 'Fresh Herbs Study with a Focus on Salmonella' last month after identifying high levels of salmonella during routine testing on imported herbs during 2006.

In light of these findings, the agency in association with Lacors started a six-month sampling study to determine the extent of the problem. Its discovery of salmonella in fresh basil samples at two Asda stores, two Sainsbury's stores and one Somerfield store, prompted the FSA's recall of fresh basil from these supermarkets last Friday.

All three chains confirmed this week that the affected batches had been taken off the shelves across the UK.

The FSA said it was conducting further investigations throughout the supply chain into the possible source of this problem to ensure no further contamination had occurred.

Twenty six cases of Salmonella senftenberg in England and Wales have been reported to the agency since the beginning of April compared with fewer than 20 cases in the past two years. Testing has also confirmed that some strains in the salmonella poisoning cases were the same as those found in the basil.

A spokesman from the HPA said it had not yet confirmed a definite link between the rise in human cases of salmonella and fresh basil, but further investigation into the issue was under way.

The results will be revealed when the study has been completed, in October.

Herbs for the culinary market have an annual value of about £30m, said a spokesman from the UK's Herb Society, and consumption was increasing 10% per year. The vast majority of herbs sold in the UK market were currently imported from Mediterranean countries, he added.

The primary market for herbs is the food manufacturing sector, which processes 50% to 60% of the products, and the secondary market is the retail and catering sector.