Fears about accidental contamination have been discussed since the first genetically modified crops were created.

Current scientific techniques mean that genetic changes made to plants are permanent.

Anti-GM campaigners have long protested about contamination

When a new gene - for instance one which will make the crop resistant to certain pests - is inserted into DNA from a plant cell, it is then found in every cell of the resulting plant.

This includes the pollen, which can spread to neighbouring fields and fertilise non-GM crops.

If this happens, new plants in that field may contain the GM gene.

Research published last week showed clearly that GM pollen can contaminate fields up to three kilometres (1.85 miles) away.

Accidental contamination

But scientists are keen to point out that every plant's pollen is different. In the case of oil seed rape - the pollen is tiny and can easily be carried by wind.

Research and pressure group Genewatch says that in Canada for instance, it is almost impossible to buy GM-free oil seed rape as farmers cannot guarantee their stocks have not been contaminated.

Accidental contamination can also happen after harvesting and is often down to human error.

For example, GM-free grain can be contaminated if it is transported in a truck which had previously held GM varieties.

The European Union says accidental contamination with genetically modified organisms is lower on organic farms because separate production and marketing channels already exist for organic produce.

In animals that have been fed GM food, the scenario is slightly different. Here, contamination of meat and dairy produce by foreign DNA is highly unlikely but not impossible.