The egg is prevented from hardening too quickly

However, the scientists, including Dr Andrea Crisanti at Imperial College London, UK, say there would need to be a full political, ethical and scientific review before any such genetically-modified animals were released into the environment.

This point was emphasised by Chris Curtis, professor of medical entomology at the London School of Hygiene and Tropical Medicine.

"I think one should have concern for the remote possibility that the modifications could make the mosquitoes able to carry a virus that they cannot carry at present," he told the BBC.

"And of course one thinks about HIV. Certainly any females - only females bite - with the transgenic technology applied to them should be tested for their susceptibility to infection by dangerous viruses before they are released. Those tests could be done, and should be done."

Circular molecules

The European team introduced a piece of foreign DNA into mosquito eggs, which then "jumped" into the chromosomes of the larvae.

Fluorescence merely indicates the technique works

The DNA, contained on circular molecules called plasmids, included the code for a fluorescent protein, which makes the mosquitoes glow an eerie green when they are exposed to ultraviolet light.

The glow has no use other than to show researchers very quickly which of their experiments have been successful. Other, more useful genes would have to be introduced to make the mosquitoes immune to the malaria parasite.

Although other insects have been modified in this way before, mosquitoes have proved especially resistant to the technique.

This is because the DNA is injected into freshly laid mosquito eggs and these harden very quickly, making micro-injection extremely difficult.

Dr Crisanti: Ethical review required

"Our secret is that we have treated the egg with a compound so that its maturation process is delayed," Dr Crisanti said. "This keeps the egg shell soft and makes it still possible to penetrate with a needle and inject the DNA."

The work was done on Anopheles stephensi, one of the major carriers of malaria in urban areas of India. It will also be tried on Anopheles gambiae, the principal carrier of malaria in sub-Saharan Africa, where 90% of all malaria cases occur - an estimated one million deaths a year.

Scientists are hopeful that genetic engineering will help them overcome some of the problems now being encountered by health agencies.

The malaria parasite is becoming resistant to the most commonly used drugs, and control of mosquito populations is being hampered by resistance to insecticides.

One of the most effective chemicals, DDT, has been withdrawn in some regions because of concerns about its long-term environmental impact.

Risks and benefits

Dr Susan Mayer of GeneWatch, which campaigns on the ethics of genetic engineering, said the risks and benefits of releasing GM mosquitoes into the wild would have to be carefully examined.

"I think as the scientists are saying these are very early stages and there is a lot more that needs to be done," she told the BBC.

"There are some very practical questions about whether you can change a population of malaria mosquitoes on a large scale, which is what you would have to do. And there will be ecological questions too.

"But malaria is an important, damaging disease and we have to find new solutions to it. This may be one way - there may be better ways. We need to take a broad look at it."