A broken protein recycling system in the brain and spinal cord may underlie all forms of motor neurone disease, scientists have learned.

The discovery could pave the way to new treatments for the devastating disorder whose victims include top physicist Stephen Hawking.

MND, also known as amytrophic lateral sclerosis, causes muscles to weaken and waste away, leading to crippling paralysis. The ability to swallow and breathe is affected, commonly resulting in death within three years.

US scientists said they had uncovered the biological key to the condition. It is a protein called ubiquilin 2 which plays a crucial role in recycling damaged or misshapen proteins in motor nerves.

Proper functioning of neurons depends on efficient recycling of proteins in the cells. When this system breaks down, the nerve cells cannot repair or maintain themselves and become severely damaged.