Twenty-one macaque monkeys were given organs from genetically modified Yucatan miniature pigs.

Scientists made 69 genetic modifications to stop the kidneys being rejected and to prolong survival.

These included eliminating pig viruses and taking away antigen coding genes that the human immune system attacks, causing the organ to be rejected.

Other modifications targeted the porcine endogenous retrovirus (PERV) gene - which research suggests could infect human cells.

The human genes added to the pigs also helped prolong the survival of the kidneys.

The organs edited only to remove antigen genes had "poor graft survival", say scientists from Harvard Medical School and biotech firm eGenesis.

However, those which also added seven human genes saw survival rates increase seven-fold - with an average 176-day survival, and one monkey survived 758 days.

Chief executive of eGenesis, Dr Michael Curtis, said it was an "extraordinary milestone" that brings testing of genetically modified pig kidneys for humans a step closer.

He said it "paves the way to better outcomes for countless individuals in need of life-saving organ transplants".

Harvard Medical School's Professor Tatsuo Kawai said the result in humans was likely to be even more favourable than was seen in the monkeys.

More than 5,000 people are currently waiting for a kidney transplant in the UK, more than three quarters of all people waiting for any kind of transplant.

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Pigs are the most suitable donor due to factors such as their size and similarities to human organs, but stopping the immune system rejection of the transplant has been a challenge for decades.

However, recent work using gene editing and techniques to suppress the immune system has been encouraging.

Two people have had a pig's heart: The first died two months after the surgery in 2022, while the second received his new organ last month.

University of Maryland Medical Centre, which performed both surgeries, said the man "continues to recover and has commenced physical therapy".

US scientists also recently put a genetically modified pig kidney into a 58-year-old man who was brain dead. The organ worked for two months - the longest achieved so far.