The gene is used by plants to take up nutritious forms of iron from the soil. The World Health Organisation estimates that 3.7 billion malnourished people suffer from iron deficiency. It is known as the "hidden hunger".

The research team's leader believes the discovery of the FRO2 gene could have far-reaching benefits despite the recent outcry in the UK over the safety of genetically-modified (GM) crops.

Professor Nigel Robinson, at Newcastle University Medical School, England, said: "This is a British example of how the genetic modification of crops could benefit consumers and the environment.

"The technology is inherently safe but each application of the technology to produce a new crop must also be assessed to ensure that it is safe."

Professor Nigel Robinson: "There's an element of xenophobia about opposition to GM crops but our local press has been great"

He explained that the nutritional quality of crops had suffered because selective breeding of plants over thousands of years had aimed solely to increase harvests. "There is now the potential to breed back into foods nutrients they have lost during selective breeding," he said.

Professor Mary Lou Guerinot, of Dartmouth College, USA, also worked on the project and said: "Because plants are the principle source of iron for most people, the generation of iron-fortified crops could have a significant impact on human health."

The scientists isolated the gene in the plant arabidopsis, or thale cress, but they believe similar genes work in all plants. Professor Robinson told BBC News Online that singling out the gene from the 30,000 in the arabidopsis genome was a painstaking and difficult task.

The right kind of iron

Iron is found naturally in two forms: ferric (3+) and ferrous (2+). Ferric iron is more common in soils but cannot be absorbed by humans.

The FRO2 gene helps plants produce an enzyme called reductase, which enables them to convert ferric iron to ferrous iron. Manipulating this gene to produce more reductase will enable plants living in poor soil to take up more of the nutritious form of iron.

The team is now carrying out further tests to see whether the increased production of iron can be concentrated in parts of the plant that humans might eat.

Iron deficiency leads to diseases like anaemia, in which the blood contains too little haemoglobin. In Ethiopia, for example, it affects 50% of women and children and 25% of men.