One of the cruellest and most devastating diseases – Huntington’s – has been successfully treated for the first time, say doctors.

The disease runs through families, relentlessly kills brain cells and resembles a combination of dementia, Parkinson’s and motor neurone disease.

An emotional research team became tearful as they described how data shows the disease was slowed by 75% in patients.

It means the decline you would normally expect in one year would take four years after treatment, giving patients decades of “good quality life”, Prof Sarah Tabrizi told BBC News.

The new treatment is a type of gene therapy given during 12 to 18 hours of delicate brain surgery.

The first symptoms of Huntington’s disease tend to appear in your 30s or 40s and is normally fatal within two decades – opening the possibility that earlier treatment could prevent symptoms from ever emerging.

Prof Tabrizi, director of the University College London Huntington’s Disease Centre, described the results as “spectacular”.

“We never in our wildest dreams would have expected a 75% slowing of clinical progression,” she said.

None of the patients who have been treated are being identified, but one was medically retired and has returned to work. Others in the trial are still walking despite being expected to need a wheelchair.

Treatment is likely to be very expensive. However, this is a moment of real hope in a disease that hits people in their prime and devastates families.

Huntington’s runs through Jack May-Davis’ family. He has the faulty gene that causes the disease, as did his dad, Fred, and his grandmother, Joyce.

Jack said it was “really awful and horrible” watching his dad’s inexorable decline.

The first symptoms appeared in Fred’s late 30s, including changes in behaviour and the way he moved. He eventually needed 24/7 palliative care before he died at the age of 54, in 2016.

Jack is 30, a barrister’s clerk, newly engaged to Chloe and has taken part in research at UCL to turn his diagnosis into a positive.

But he’d always known he was destined to share his father’s fate, until today.

Now he says the “absolutely incredible” breakthrough has left him “overwhelmed” and able to look to a future that “seems a little bit brighter, it does allow me to think my life could be that much longer”.