There is significant variation across the UK in the number of organs donated for transplant, research suggests.
More studies are needed into why this happens before any new law for presumed consent is introduced in the UK, experts said.
Their study found that England had significantly fewer heart donations than both Wales and Northern Ireland. However, the rate of heart donations has fallen over the past 20 years for Wales, Scotland and England, they said.
While there were no real differences in lung donations, Northern Ireland has "consistently lower" donation rates involving the cornea than England, Scotland or Wales. Overall, heart and liver donations were highest in Wales and Northern Ireland.
The researchers looked at organs transplanted after the owner's death, using data from NHS Blood and Transplant for all four UK countries for the period 1990 to 2009.
They found that since the organ donor register was first launched in 1994, Wales has "consistently outperformed" other parts of the UK in terms of the percentage of population registered, with Scotland in second position, England third and Northern Ireland last.
The rate of organ donation is also consistently higher in Wales than the UK average for the majority of the last 20 years.
Donation rates in Northern Ireland are the second highest (despite a lower proportion of people on the register than Scotland or England), achieving rates higher than the UK average in 13 of those years. Scotland achieved this in six of these years but England only managed it in three.
Writing in the British Medical Journal, researchers from the University of Ulster said: "While registration and donation are both highest in Wales, among other parts of the UK registration and donation do not follow similar trends."
"Further exploration of underlying factors, organisational issues, practices and attitudes to organ donation in the four regions of the UK, taking into account findings from EU countries with varying approaches to presumed consent, needs to be undertaken before such legislation is introduced across the UK."