Researchers used more than a 100 spare embryos left over from treatment at fertility clinics to establish several embryonic stem cell “lines”.

One of those lines, known as RC-7, was transformed into blood stem cells before they were converted into red cells containing haemoglobin, the oxygen-carrying pigment.

Scientists say the aim of the £3million research was to find cells genetically programmed to develop into the O-negative blood group.

This is the universal donor group whose blood can be transfused into anyone without fear of tissue rejection, but is only found in seven per cent of the population.

Researchers also said the aim of the project, funded by the Wellcome Trust, the charity, was to establish a manufacturing process to produce more than two million pints a year.

Experts said the research could soon revolutionise medicine by ending the need to rely on volunteer donors to provide a constant source of blood for lifesaving transfusions.

The synthetic version would also be guaranteed free of infections, such as the human form of "mad cow" disease.

It also raises the possibility of producing the entire supply of blood for the UK from a single "universal blood donor" who only ever existed as a four-day-old IVF embryo.

A key requirement for synthetic blood is for it to be cheaper than donated blood to produce.

At present it costs about £180 to make a pint of donated blood ready for transfusion, but with extra costs for testing and donation it is closer to £500 per pint.

Blood supplies face a constant threat of shortages, with more than 2.5million donations needed annually for transfusions and blood products.

In order to achieve regulatory approval, the project has reportedly produced four embryonic stem cell lines suitable enough for human clinical trials, which scientists said conformed to the "good manufacturing practice" (GMP) grade.

Prof Marc Turner, of Edinburgh University, who is also clinical director of the Scottish National Blood Transfusion Service (SNBTS), said synthetic blood could transform medicine if made on industrial scale.

It could overcome the problem of blood shortages while at the same time reducing the risk of transmitting new infections between donors and recipients.

"We've proved the principle that from these embryonic stem cell lines we can generate red blood cells," Prof Turner, who led the project, told The Independent.

"The regulators, the Medicines and Healthcare Products Regulatory Agency and the European Medicines Agency, would need to be comfortable that the cells were produced to sufficient quality and safety before they would allow the first studies in humans.”

“If we can crack it with red cells, it takes us a long way. It doesn't solve it with other tissues but it takes us a long way to cracking it with other tissues, such as liver and so on."

Jo Mountford, a stem cell scientist at Glasgow University, added: “We have cells that are clearly red so we're happy with that.

“We've managed to go 90 per cent down the path towards fully differentiated, adult red blood cells.

But some experts expressed concern when the trial was first reported last year.

“Many good people who donate cord blood, adult blood or bone marrow, would not wish to be in anyway associated with this proposed destructive use of embryos, let alone research involving tissue from aborted babies,” said Josephine Quintavalle, director of the Campaign for Reproductive Ethics.

The first clinical trials of synthetic blood made from embryonic stem cells could begin within five years