A British hospital has made the world’s first attempt to treat blindness with a revolutionary gene therapy.

Surgeons at the Moorfields Eye Hospital in London operated on Robert Johnson, who was born with a rare sight disorder known as Leber’s congenital amaurosis (LCA), which deteriorates with age.

Mr Johnson, 23, who had genes inserted into one eye, could see only outlines during the day and very little at night before having the procedure yesterday. He is one of a dozen young patients selected for the first clinical trial to test the new therapy, which has already proved successful at restoring the sight of dogs in tests.

It will be months before the researchers know whether their work has been a success, but it is thought that the therapy could be used to treat a wide range of inherited sight disorders in adults and children.

The LCA disorder is caused by a defect in a gene called RPE65, which prevents the light-sensitive layer of cells in the retina at the back of the eye from working properly.

Usually these are cells that detect light, but in Mr Johnson’s case they are damaged and prevent him from seeing properly.

The operation, conceived by researchers from University College London, involved injecting working copies of the defective gene into the back of the eye. Surgeons used a harmless virus or “vector” to carry the gene into the cells.

It is hoped that the replacement genes will enable the retina to detect light – and eventually restore Mr Johnson’s sight.

The trial, funded by the Department of Health, involves 12 adults and children with LCA, for which there are currently no effective treatments.

During preliminary studies, the vision of dogs with the defect was restored to the extent that they were able to walk through a maze without difficulty; something they could not do before the treatment.

The purpose of the Moorfields trial is to find out how safe and effective the intervention is for humans.

The researchers hope that their work could lead to ways to treat more common sight problems, such as age-related macular degeneration, which affects about 250,000 Britons.

Most previous gene therapies have been developed in an attempt to treat different types of cancer.

Before surgery, Mr Johnson told the BBC that he had mixed feelings.

He said: “It’s very difficult to say how I’m feeling. I keep ranging from extreme nervousness to a bit of excitement.”

Professor Robin Ali, the lead researcher, based at the Institute of Ophthalmology, has spent 15 years working with colleagues developing the technique. He said yesterday: “I can’t help feeling somewhat apprehensive. There is so much riding on it and we have all been waiting for a very long time.”

His colleague, James Bainbridge, who carried out the surgery, said that there was no guarantee that it would be a success. However, he added: “It is very encouraging that we can deliver genes to an extremely fragile site in the eye without complications.”

The surgery required incredible precision. Robert Maclaren, the assistant surgeon, said yesterday that he was pleased with how things went. “We couldn’t have asked for a better result,” he said.

Professor Ali added: “There are many forms of retinal degeneration, meaning the use of gene therapy treatments must be individually developed, then tested in a separate clinical trial specifically for that disease.

“However, the results from this first human trial are likely to provide an important basis for many more gene therapy protocols in the future.”

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