The ageing process has been biologically reversed for the first time by giving humans oxygen therapy in a pressurised chamber.
Scientists in Israel showed they could turn back the clock in two key areas of the body believed to be responsible for the frailty and ill health that come with getting older.
As people age, the protective caps at the ends of chromosomes - called telomeres - shorten, causing DNA to become damaged and cells to stop replicating. At the same time, "zombie" senescent cells build up in the body, preventing regeneration.
Increasing telomere length and getting rid of senescent cells are the focus of many anti-ageing studies.
Now, scientists have shown that giving pure oxygen to older people while in a hyperbaric chamber increased the length of their telomeres by 20 per cent, a feat never before achieved.
Researchers said the growth might mean participants' telomeres were as long as they had been 25 years earlier.
The therapy also reduced senescent cells by up to 37 per cent, making way for healthy cells to regrow. Animal studies have shown that removing senescent cells extends remaining life by more than one third.
"Since telomere shortening is considered the 'Holy Grail' of the biology of ageing, many pharmacological and environmental interventions are being extensively explored in the hopes of enabling telomere elongation," Professor Shai Efrati, of Tel Aviv University, said.
"The significant improvement of telomere length shown during and after these unique protocols provides the scientific community with a new foundation of understanding that ageing can, indeed, be targeted and reversed at the basic cellular-biological level."
Many scientists believe ageing itself is responsible for major conditions such as Alzheimer's, Parkinson's, arthritis, cancer, heart disease and diabetes.
It is also known that obesity, smoking, lack of physical activity, vitamin deficiency and inflammation can speed up the shortening of telomeres, demonstrating that they have a major impact on longevity.
During the trials, 35 healthy adults aged 64 and over were placed in a hyperbaric chamber for 90 minutes for five days a week over three months while breathing 100 per cent oxygen through a mask.
Dr Amir Hadanny, chief medical research officer at Israel's Sagol Centre for Hyperbaric Medicine and Research, said: "Until now, interventions such as lifestyle modifications and intense exercise were shown to have some inhibition effect on the expected telomere length shortening.
"However, what is remarkable to note in our study, is that in just three months of therapy, we were able to achieve such significant telomere elongation - at rates far beyond any of the current available interventions or lifestyle modifications."
The research was published in the journal Ageing.