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The ability to breathe has been restored to mice with spinal cord injuries, in what US researchers describe as a medical first.

Some patients with damaged spinal cords need ventilators as they are unable to breathe on their own.

A report in the journal Nature showed a nerve graft, coupled with a protein, could restore breathing.

Human trials could begin soon, which the charity Spinal Research said could be "potentially life-changing".

Damage at the top of the spinal cord, around the neck, can interrupt messages to the diaphragm - a layer of muscle involved in breathing.

The cord is notoriously resistant to repair. Techniques such as nerve grafts, which worked in the arms and legs, had shown limited success with the spinal cord, doctors at the Case Western Reserve University said.

The spinal cord scars after it is damaged, and molecules - chondroitin sulphate proteoglycans - prevent nerves repairing and forming new connections.

The researchers used a nerve graft to form a bridge across the scar at the same time as injecting an enzyme - chondoitinase ABC - which attacked the inhibitory molecules.

Three months later, tests showed the mice had recovered 80-100% of breathing function.

Professor of neuroscience and lead researcher Jerry Silver said: "The use of the enzyme, that's helped get the nerve fibres out and we were amazed at, once they get out, how well they can reconnect.

"The spinal cord can just figure things out and restore really beautiful functional breathing patterns."

Researchers hope to begin trials in humans. They are also investigating whether bladder function can be restored, which can be lost when the lower spine is damaged.

Dr Mark Bacon, from the charity Spinal Research, said: "Long distance regeneration has remained quite elusive in the field of spinal cord injury repair, so to achieve this and at the end of it establish functional connections that actually do something useful - restore breathing - is remarkable.

"It is potentially life-changing if this or similar techniques can be translated to the clinic."