Scientists have found a way to harvest more stem cells from a baby's umbilical cord.

It could help in pioneering transplant operations to treat adults with leukaemia.

The umbilical cords of babies are a rich source of stem cells, the "master cells" of the body.

They have been used as an alternative to bone marrow transplants to treat more than 2,000 children around the world.

There is often too little blood, however, to make it a safe way of treating adults and only 500 transplants have been carried out.

Researchers at the Fred Hutchinson Cancer Center in Seattle, US, have been able to increase the yield of stem cells from cord blood more than a hundred times.

They did this by growing up the cells in the laboratory in the presence of a growth substance known as Delta 1.

Tests on mice with damaged immune systems suggest the technique could be useful for providing more stem cells for human transplant operations.

'Clever science'

In July this year, a leukaemia sufferer became the first adult in the UK to undergo a bone marrow transplant using blood from a baby's umbilical cord.

Stephen Knox, aged 31, underwent the treatment after being given just months to live.

The procedure was developed by Professor Stephen Proctor, who used stem cell blood from discarded placentas and umbilical cords.

Prof Proctor, based at Newcastle Hospital Trust's Haematology Unit and at Newcastle University, said the latest research was promising.

"It's clever science, it's clever technology but it adds another layer of expense to transplant operations," he told BBC News Online.

The US research is published in the Journal of Clinical Investigation.