The first patients to receive a therapy based on embryonic stem cells are people who do not yet know that they will need it.

The paralysis treatment that has been cleared to begin clinical trials is designed to work best in the first two weeks after a spinal cord injury — so those who may benefit are fit and healthy at the moment.

Over the next few months, however, people who become paralysed from the chest down as a result of road or sporting accidents could be treated with the stem-cell therapy developed by the Geron Corporation.

If the trials at seven centres across the US go well, the treatment could be extended to patients with neck injuries similar to those sustained by Christopher Reeve, the Superman actor who was paralysed after being thrown from his horse. He went on to become an outspoken campaigner for stem-cell research before his death in 2004.

The therapy is unlikely, however, to prove suitable for people who are already paralysed: animal experiments suggest that it is ineffective if administered more than three months after injury.

The Geron treatment has been developed using stem cells taken from human embryos, which have then been coaxed to developed into a specialised type of cell known as oligodendrocyte progenitor cells (OPCs). These will be injected into the patients’ spines, at the spot where they are injured, between 7 and 14 days after they become paralysed.