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Researchers at Geron Bio-Med believe the piglet will arrive by early 2000 at the latest.

They hope eventually to modify the technology to make other pig clones whose organs can be transplanted into humans.

The first stage of a cloned pig embryo

There are many more patients needing a transplant than there are donor organs available.

Scientists believe pigs could fill this tissue gap, provided the problems of rejection can be overcome and the dangers of transferring animal viruses to humans are eliminated.

Geron Bio-Med thinks its cloning technology is the key to producing animal tissue that is safe to use in humans.

Deactivating genes

"What you want to be able to do for xenotransplantation - for production of organs - is to remove or deactivate genes that cause immunological problems," said Simon Best, the company's Managing Director.

"Cloning is the first technology that allows you to take away genes as opposed to adding them."

Simon Best says cloning can make pig tissue safe for use in humans

The genes that code for chemicals which trigger rejection in humans would need to be knocked out. And, crucially, the genes that code for pig retroviruses would also have to be removed.

However, the company, a commercial off-shoot of the Roslin Institute which produced Dolly the Sheep, says it still has some way to go before it perfects the basic cloning technology that will allow it to make a living copy of a pig.

Although the cloning research has got as far as producing embryos, none of these have survived to term in their surrogate mothers. Pigs give birth in litters of four or more and it seems the normal embryos will not accept an implanted clone.

Human stem cells

But chief scientific officer Professor Ian Wilmut, who helped to develop the cloning technology known as nuclear transfer, is hopeful of seeing cloned piglets "later this year or early in the new millennium".

Simon Best: The future lies in combining cloning with stem cell technology

He said: "We've recently got to the stage where we can produce embryos by nuclear transfer for the first time.

"We are transferring them into recipients in order to monitor their development for the first week, but it is clear we have a lot more research still to carry out."

Geron Bio-Med is a joint venture between the scientists who cloned Dolly the Sheep and the big American biotech company Geron, which has done pioneering work in the field of stem cell technology.

Stem cells are the "master" cells for all the tissues in the body. If scientists can control the way these cells develop, they could, theoretically, grow any type of tissue they desired in the lab.

And combined with cloning, it would then be possible to create perfect-match tissue to treat people with diseases such as leukaemia.