Image copyright Norfolk Plant Sciences Image caption The new tomatoes could improve the nutritional value of everyday foods

The prospect of genetically modified purple tomatoes reaching the shelves has come a step closer.

Their dark pigment is intended to give tomatoes the same potential health benefits as fruit such as blueberries.

Developed in Britain, large-scale production is now under way in Canada with the first 1,200 litres of purple tomato juice ready for shipping.

The pigment, known as anthocyanin, is an antioxidant which studies on animals show could help fight cancer.

Scientists say the new tomatoes could improve the nutritional value of everything from ketchup to pizza topping.

The tomatoes were developed at the John Innes Centre in Norwich where Prof Cathie Martin hopes the first delivery of large quantities of juice will allow researchers to investigate its potential.

"With these purple tomatoes you can get the same compounds that are present in blueberries and cranberries that give them their health benefits - but you can apply them to foods that people actually eat in significant amounts and are reasonably affordable," she said.

I hope this will serve as a vanguard product where people can have access to something that is GM but has benefits for them Prof Cathie Martin, John Innes Centre in Norwich

The tomatoes are part of a new generation of GM plants designed to appeal to consumers - the first types were aimed specifically at farmers as new tools in agriculture.

The purple pigment is the result of the transfer of a gene from a snapdragon plant - the modification triggers a process within the tomato plant allowing the anthocyanin to develop.

Although the invention is British, Prof Martin says European Union restrictions on GM encouraged her to look abroad to develop the technology.

Canadian regulations are seen as more supportive of GM and that led to a deal with an Ontario company, New Energy Farms, which is now producing enough purple tomatoes in a 465 square metre (5,000sq ft) greenhouse to make 2,000 litres (440 gallons) of juice.

According to Prof Martin, the Canadian system is "very enlightened".

"They look at the trait not the technology and that should be a way we start changing our thinking - asking if what you're doing is safe and beneficial, not 'Is it GM and therefore we're going to reject it completely'.

"It is frustrating that we've had to go to Canada to do a lot of the growing and the processing and I hope this will serve as a vanguard product where people can have access to something that is GM but has benefits for them."

The first 1,200 litres are due to be shipped to Norwich shortly - and because all the seeds will have been removed, there is no genetic material to risk any contamination.

Image copyright Rothamsted Research Image caption Scientists at Rothamsted hope to produce a GM plant that provides "fish oil"

The aim is to use the juice in research to conduct a wide range of tests including examining whether the anthocyanin has positive effects on humans. Earlier studies show benefits as an anti-inflammatory and in slowing cancers in mice.

A key question is whether a GM product that may have health benefits will influence public opinion.

A major survey across the European Union in 2010 found opponents outnumbered supporters by roughly three to one. The last approval for a GM food crop in the EU came in 1998.

Prof Martin hopes that the purple tomato juice will have a good chance of being approved for sale to consumers in North America in as little as two years' time.

She and other plant researchers in the UK hope that GM will come to be seen in a more positive light.

Legacy of distrust

Earlier on Friday, scientists at Rothamsted Research in Hertfordshire announced that they were seeking permission for field trials for a GM plant that could produce a "fish oil".

In a parallel project, they have been cultivating a type of GM wheat that is designed to release a pheromone that deters aphids.

Professor Nick Pidgeon, an environmental psychologist at Cardiff University, has run opinion polls and focus groups on GM and other technologies.

He says that a legacy of distrust, including from the time of mad cow disease, will cause lasting concern.

"Highlighting benefits will make a difference but it's only one part of the story which is quite complex.

"People will still be concerned that this is a technology that potentially interferes with natural systems - they'll be concerned about big corporations having control over the technology and, at the end of the day, you feed it to yourself and your children and that will be a particular concern for families across the UK."

"To change that quite negative view that people had 10-15 years ago will take quite a long time - it'll take a demonstration of safety, a demonstration of good regulation and of the ability to manage the technology in a safe way. And that doesn't happen overnight."