The transplant is often a last-ditch "all or nothing" attempt to treat the cancer.

At present, terminally-ill cancer sufferers may not be able to undergo the treatment because doctors think they are already too ill, weak or old to survive the powerful chemotherapy needed to remove their own immune response so it does not reject the new marrow.

But a team of US doctors has successfully treated two terminally-ill patients with a process that removes the need for such high doses of chemotherapy.

The technique also means that more patients without identically-matched bone marrow donors can receive treatment.

The Lancet reported doctors at Massachussets General Hospital in Boston successfully treated two patients who were terminally-ill with Hodgkins Lymphoma, a cancer of the body's lymphatic system.

Only a quarter currently succeed

A bone marrow transplant is often needed to treat the disease, but currently fewer than 25 per cent of these are successful.

High dose chemotherapy injections prior to many bone marrow transplants

As well as the high-dose chemotherapy, there is also the risk of "graft-versus-host" disease which happens when the natural defence mechanism of the donor marrow treats the recipient's body like a "foreign" invader and attacks it.

The new technique involves blending together the immune systems of donor and host which both removes the need for the chemotherapy, as it reduces the risk of this rejection.

Prof Adrian Newland, a consultant haematologist from the Royal London Hospital in Whitechapel, London, said that the "mini bone marrow transplant" technique held a lot of promise.

"It reduces many of the side-effects associated with bone-marrow transplantation. Most transplants we do follow high-dose treatments.

"A lot of early work involves older patients or those who are more ill, but it could be used on a wider number of patients. It's a breakthrough."

Dr Megan Sykes, one of the Boston researchers, said: "We can achieve a lasting combination of donor and recipient immune systems without the kind of toxic treatment usually used to wipe out the recipient's bone marrow."

Of the five terminally-ill patients in the study, two are now in "long-term remission", meaning they are currently free of disease. Two died from the disease and one for another reason.