Kidney patients needing a transplant could be facing a postcode lottery in accessing organs, research out today suggests.
A study found large variations across the UK that could not be explained by differences in the health of patients.
Some kidney patients fall off the waiting list if they become too ill for a transplant but, if their health improves, they can go back on.
The average waiting time for an adult kidney transplant is 841 days but many people die before they receive an organ.
Children, who are prioritised, wait an average of 164 days.
There are currently 6,865 adults and 111 children on the UK waiting list for a kidney transplant.
Today's study, from the UK Renal Registry and NHS Blood and Transplant (NHSBT), based in Bristol, found variations in accessing the list, the time patients had to wait to get on the list, and the time they had to wait for a transplant.
The research, published online in the British Medical Journal (BMJ), studied 16,202 dialysis patients from 65 renal centres across the UK.
Data on the patients, who were all under 65, was submitted to the renal registry between January 2003 and December 2005, with follow-up to December 2008.
The national average percentage of patients registered for a transplant within two years of starting dialysis was 50%.
But the study found this varied from about 25% in some centres to 65% in others.
The research also found 25% of patients on average received a kidney from a person who suffered a cardiac death, or from a living donor, within two years of being on the list.
But this ranged from less than 10% at some centres to above 40% in others.
This kind of transplant is "predominantly locally resourced and allocation of organs is controlled by local centres' policies", the research said.
The study also found significant variation between centres on times for inclusion on the national transplant waiting list.
Time to get on the list is important because patients who are put on earlier tend to receive a transplant quicker than those listed later on.
The study found wide variations in getting on the waiting list depending on age.
Those aged 18 to 29 were most likely to get on the list, with levels falling as patients got older.
People from ethnic minorities were also less likely to get on the list, as were those with diabetes.
While some of this variability may be explained, the authors said "significant inter-centre differences should not exist in the time taken to activate suitable patients for transplantation or in receipt of a transplant once on the waiting list".
Almost a quarter (24%) of patients nationally receive a kidney from a brain dead donor within two years of being registered, but this varied from 6% to 42%.
However, the study noted centres were not to blame for this variability as there is a national allocation policy which determines where these organs go.
There are around twice as many organ transplants from brain dead donors as cardiac death donors each year.
The study found that increasing age, ethnicity and being female negatively affected access to brain dead donors.
The experts concluded: "Significant variation in access to renal transplantation exists between centres within the UK that cannot be explained by differences in case mix.
"Further work needs to be undertaken to understand whether the observed differences in centre performance are due to variations in resource availability or because certain centres have more organised and efficient patient pathways."
James Neuberger, associate medical director of organ donation and transplantation at NHSBT, welcomed the study.
"We encourage renal centres to take note of its findings so as to help bring about further improvement in care, treatment and equity.
"NHSBT is committed to ensuring there is equity of access to transplantation of all organs, regardless of various factors such as the patient's age, gender or ethnicity, and our policies are continuously reviewed to ensure they are appropriate and up-to-date."
Mr Neuberger said there had been a 40% increase in the number of kidney transplants taking place in the UK in the past five years - up from 1,855 in 2005 to 2,599 in 2009.
But he said more needed to be done.
"There is a serious shortage of donated kidneys and other organs for transplantation which leads to around 1,000 deaths every year in the UK of people who are waiting or who become too ill and have to be removed from the list."
A Department of Health spokeswoman said: "Patients who need kidney transplants deserve a high quality service and support to help them manage their transplant and achieve the best possible quality of life, regardless of where they live.
"The number of successful kidney transplants is increasing and improvements have been made to the quality of patient care, including establishing a network of donor transplant co-ordinators and appointing clinical leads for organ donation within hospital trusts.
"But we know there is more to do. We want the NHS to use this research to help deliver better access to transplants and further improve the quality of care for kidney patients."