A survey by the Austrian Consumers' Association says that the extra price paid by shoppers for fresh produce may not always be worthwhile in health terms.

The report said that some winter crops it tested were higher in nitrates - which can be converted into nitrites in the body. Although there is no strong evidence linking these to ill-health, experts believe that our consumption of nitrates should be kept in check.

Conrad Brunnhofer, the researcher involved in the study, told BBC News Online that nitrate levels were more than seven times higher in some fresh vegetable samples compared with frozen. He said: 'There is more than one reason why I would prefer frozen vegetables to fresh vegetables out of season. The problem of nitrates is one of these.' Previous research from the association has pointed to a reduction in vitamins in vegetables imported during the winter months. Any vegetable will start to lose nutrients from the moment it is harvested.

However, while the freezing process can take place within hours of the vegetables being picked, it may be some time before fresh vegetables reach supermarkets in the UK.

The association found that vegetables such as broccoli lost significant amounts of nutrients when imported fresh, and ended up less nutritious than their frozen equivalent.