The condition, which puts sufferers at risk of excessive bleeding even from slight injuries, severely affects about 2,000 people in the UK.
It can also cause life-threatening internal bleeding, which in turn can lead to joint damage and arthritis, and there is no known cure for the condition.
Sufferers of the hereditary condition, which mainly impacts men, “have virtually none” of the protein factor VIII which is needed for blood to clot.
The trial, which was led by Barts Health NHS Trust, saw 13 patients injected with a copy of the missing gene, which allowed cells to produce the missing factor.
All 13 patients were able to stop regular treatment for the condition and 11 have got “normal or nearly normal” levels of the missing protein factor after having their progress followed for 19 months.
Jake Omer, 29, of Billericay, was diagnosed with haemophilia when he was two and has had frequent injections of factor VIII to prevent bleeds ever since.
The father-of-two said: “The gene therapy has changed my life. I now have hope for my future. It is incredible to now hope that I can play with my kids, kick a ball around and climb trees well into my kids’ teenage years and beyond.
“It’s really strange to not have to worry about bleeding or swellings. The first time I noticed a difference was about four months after the treatment when I dropped a weight in the gym, bashing my elbow really badly.
“I started to panic thinking this is going to be really bad, but after icing it that night I woke up and it looked normal. That was the moment I saw proof and knew that the gene therapy had worked.”
The team will now hold further tests to include people in the USA, Europe, Africa and South America.
Professor John Pasi, Haemophilia Centre Director at Barts Health NHS Trust, said: “We have seen mind-blowing results which have far exceeded our expectations.
“When we started out we thought it would be a huge achievement to show a five per cent improvement, so to actually be seeing normal or near normal factor levels with dramatic reduction in bleeding is quite simply amazing.
“We really now have the potential to transform care for people with haemophilia using a single treatment for people who at the moment must inject themselves as often as every other day. It is so exciting.”