The patient Robert Johnson was born with Leber's congenital amaurosis (LCA), a type of inherited childhood blindness caused by a single abnormal gene; having the disorder means he can see very little at night, and he can expect his sight to deteriorate even further with age.

The team say it will be several months before the researchers know whether their work with Mr. Johnson and 11 other has been a success but the doctors say there have been no complications so far.

LCA is an inherited degeneration of the retina and the condition prevents the retina from detecting light properly, resulting in progressive deterioration and severely impaired eyesight. There is currently no effective treatment.

In this form of retinal degeneration the photoreceptor cells are present, but not functioning.

The aim is to restore the activity in the photoreceptor cells and therefore restore vision by implanting healthy copies of the key gene into the RPE65 gene into cells of the retina using a harmless virus or vector.

The team of doctors are collaborating with Seattle, Washington-based biotech firm Targeted Genetics which made the vector being used in the trial.

Testing the procedure on humans follows 15 years of laboratory and animal experimentation, including tests on dogs whose vision was restored to the extent they could navigate a maze with ease.

Robin Ali, professor of human molecular genetics at UCL says testing the technique for the first time in patients is an important and exciting step towards establishing gene therapy for the treatment of many different eye conditions.

Funding for the trial to the tune of 1 million pounds was given by the Department of Health, which says the research is pioneering and underlines Britain's leading position in gene therapy in Europe.

Using gene therapy to fix diseases caused by genetic faults is not a new idea although getting it to work in practice has proven complex.

The majority of gene therapy trials to date have been for cancer, where the process is complicated by the need to reach multiple sites in the body.

The eye is however relatively straightforward and the researchers hope that their work could lead to ways to treat more common sight problems, such as age-related macular degeneration.