By Jeremy Cooke BBC New York Correspondent

The family's billboard advert

But some patients in the US are hoping to bypass the system.

Jorge Miranda, 44, who has advanced liver disease, was told three years ago he had three years to live.

He is on the waiting list for a liver transplant, but is making a direct appeal for an organ using billboards around his home town of Chicago.

Jorge's liver was punctured in 1976 and he received a blood transfusion. In 2001, the family learnt he had contracted an extremely vigorous strain of Hepatitis C.

The family is using money provided by an anonymous donor to fund the adverts.

Mr Miranda told the BBC: "My main objective is to be here for my children, my wife. I have always been a provider.

"To see those things dwindling away is pretty scary."

'I'd do anything'

But doctors are concerned that such direct appeals subvert the existing voluntary donor system, where recipients of donor organs are prioritised because of factors including need, the length of time they have spent on the waiting list and age.

There are fears that a successful advert may mean that someone on the official list, who is sicker and who needs the organ more, may miss out.

Dr Stewart Greenstein, of the US Organ Procurement and Transplantation Network (OPTN), said: "You may advertise that you need a liver, but you may not be the sickest person on the list."

In a statement, the OPTN added: "We strongly oppose public or private appeals that effectively put the needs of one candidate above all others and pose concerns of fairness.

"Transplant candidates rely on the public's trust in the fairness of the allocation system and support of that system through donation. Public appeals may jeopardise that trust."