The carbon footprint of lower-yielding tomato varieties is typically twice that of higher-yielding varieties such as classic loose tomatoes, new research has shown.

A project commissioned by the Horticultural Development Council (HDC) and carried out by Bangor University, found yield was a key factor in ­determining the level of greenhouse gas emitted in the production of tomatoes. Also important were the age of the greenhouse and the type of boiler used to heat it.

The differences in footprint these factors could produce were considerable, said Gareth Edwards-Jones, professor of agriculture and land use studies at Bangor. Across all varieties, there was a 14-fold difference in the carbon footprint created by the best-performing British grower captured by the study and the worst-performing, he added. This showed how difficult it was to make accurate statements about the carbon impact of British tomatoes.

The actual carbon footprint figures will not be released publicly. Carbon footprinting was a sensitive issue for growers, who had become wary of media exposure after seeing their fruit depicted as a high-carbon food, said Edwards-Jones. "There have been various press comparisons recently between British and Spanish tomatoes, which have suggested Spanish tomatoes have the lower footprint. Growers feel these calculations haven't always been fair, so they are wary of ­declaring what their footprint is."

The new study did not compare British and Spanish supply chains, and Edwards-Jones would not be drawn on whether the results indicated British ­tomatoes had a smaller or bigger footprint than their Spanish counterparts.

It is now up to individual growers to communicate the exact findings of the study to their supermarket customers. The supermarkets had shown interest in seeing the figures, said Edwards-Jones, but many "don't really know what to do with them. It's a private negotiation now about what happens next."