A treatment that has the potential to reverse the effects of multiple sclerosis has been tested in clinical trials in four countries.

A stem cell treatment for people with multiple sclerosis (MS) has enabled some patients to walk again, doctors say.

About 20 patients have received bone marrow transplants using their own stem cells in a clinical trial at Sheffield's Royal Hallamshire Hospital, which is also being run in the US, Sweden and Brazil.

Some patients who were paralysed have been able to walk again.

Professor Basil Sharrack, from the hospital, told the BBC Panorama program: "To have a treatment which can potentially reverse disability is really a major achievement."

The treatment - known as an autologous haematopoietic stem cell transplant (HSCT) - aims to destroy the faulty immune system using chemotherapy.

It is then rebuilt with stem cells harvested from the patient's own blood.

Professor John Snowden, consultant haematologist at Royal Hallamshire Hospital, said: "The immune system is being reset or rebooted back to a time point before it caused MS.

"It's clear we have made a big impact on patients' lives, which is gratifying."

More than 100,000 people in the UK have MS, which is an incurable neurological condition.

One patient, Steven Storey, told the BBC: "I went from running marathons to needing 24-hour acute care. At one point I couldn't even hold a spoon and feed myself."

Within a few days of the transplant he was able to move his toes, and after four months he could stand unaided.

He still needs a wheelchair but is astounded at his progress: "It's been incredible. I was in a dire place, but now I can swim and cycle and I am determined to walk."

Holly Drewry was 21 when she was diagnosed with MS and her condition deteriorated after she gave birth to her daughter Isla.

Ms Drewry needed a wheelchair before her transplant, but after the treatment she walked out of hospital.

Dr Emma Gray, head of clinical trials at UK's MS Society, said: "Ongoing research suggests stem cell treatments such as HSCT could offer hope, and it's clear that, in the cases highlighted by Panorama, they've had a life-changing impact.

"However, trials have found that, while HSCT may be able to stabilise or improve disability in some people with MS, it may not be effective for all types of the condition."