What is GM food?
Genetically modified food is produced from plants which have had their genetic make-up tweaked in the lab.
Scientists "cut and paste" a gene from another organism into a plant's DNA to give it a new characteristic.
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This can be to increase yield or to allow the plant to exist in a more hostile environment than normal.
Pro-GM scientists say this means cheaper more plentiful food but opponents argue we do not know the consequences of meddling with nature
How does the process work?
The first step in GM is identifying a gene for a particular characteristic, such as herbicide resistance.
The gene, which may come from any other organism, is inserted into the DNA of a plant cell, giving it the same trait.
Maize is one of the crops that has been genetically modified
This means, for example, that a field can be sprayed with weedkiller and the GM crops will be unaffected.
Why is it controversial?
Farmers have modified their crops for thousands of years by crossing similar species.
However, modern GM is controversial because critics say the modified crops could "escape" and cross with wild plants, with unknown consequences.
They also argue that more chemicals are used on some GM fields which may have a negative impact on other plants and wildlife.
And while no study has found GM food to be harmful to humans, opponents say it is too soon to be sure.
Are there any benefits?
Supporters of GM say there is no evidence that modified crops cause illness in humans.
They say the regulatory structures that govern GM foods are in many ways far stricter than for conventional foods.
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GM crops can be enriched with nutrients and they produce bigger yields and therefore use farmland more efficiently.
As for environmental concerns, GM crops also use fewer agro-chemicals such as pesticides.
But people in the UK have become so concerned about GM foods that most supermarkets have banned them.