Animal organs could be regularly transplanted into human beings within 20 years, according to a leading UK scientist.
Using a combination of genetic modification and new drugs, a pig's heart has been kept beating in another animal for more than a year in the US.
In controversial experiments, Dr Muhammad Mohiuddin at the US medical research agency the NIH transplanted a pig's heart into a baboon and kept it beating for a year.
Dr Mohiuddin said his research will "instill a new ray of hope for thousands of patients waiting for human donor organs".
Professor Chris Mason, from the department of biochemical engineering at University College London, said the experiments were a major breakthrough that could pave the way for research that could save the lives of thousands.
"It is very early, it is not in man and it's not even in the position of a heart, but it's a huge step forward," he said.
In the UK, approximately 1,000 people die each year while waiting for an organ transplant, and Professor Mason said that it may soon be possible to farm pigs to transplant their organs on demand.
"We are talking 10, 15 or more years away," he told Sky News.
"This is early proof of the concept that shows that a pig's heart can be transplanted into a non-human primate and not be rejected."
Currently, those on waiting lists use an artificial heart, but these are prone to power supply problems, blood clotting, infection and haemolysis, the breakdown of red blood cells.
Scientists believe that pigs' organs could be best for transplant as they are compatible with human organs.