The move, which comes after five years of research, is seen as the penultimate stage in the endeavour to create an artificial life form based entirely on a man-made DNA genome - something that has tantalised scientists and sci-fi writers for decades.

Scientists had previously constructed the complete DNA of viruses, but this is the first time it has been done for bacteria, which are far more complex.

The artificial genome is more than 10 times as long as the longest piece of DNA ever synthesised.

The feat is a watershed for an emerging field called synthetic biology, which involves the design of organisms to perform particular tasks, like making bio-fuels.

Synthetic biologists envision being able to design an organism on a computer, pressing the 'print' button to have the necessary DNA made and then putting that DNA into a cell to produce a custom-made creature.

'What we are doing with the synthetic chromosome is going to be the design process of the future,' said maverick boundary-pushing gene scientist J. Craig Venter, whose Maryland-based institute created the artificial DNA.

His team's work was published online on Thursday by the journal Science.

DNA is composed of chemicals. Working from the DNA blueprint of a natural bacterium called mycoplasma genitalium, which causes certain sexually transmitted diseases, the team created chemical fragments of DNA and pieced them together to make a copy of the original bacterium.

Dr Venter said the goal was to design novel microbes whose handcrafted genomes endow them with the ability to produce useful chemicals, including renewable synthetic fuels that could substitute for oil.

But there are concerns that synthetic biology could be used to make pathogens, or that errors by well-intended scientists could produce organisms that wreak environmental havoc.

In any case, there are many hurdles to overcome before Dr Venter's vision of 'life by design' is realised.

The synthetic genome made by his team was only a copy of the bacterium. It was not designed from scratch.

Moreover, Dr Venter's team has yet to accomplish the next and biggest step - to transplant the synthetic genome into another cell so that it can use the existing machinery to 'boot up' and start growing and reproducing.

Such a potentially historic event would amount to the creation of the first truly artificial life form.

The team's failure to achieve that tempered the reaction of some scientists.

Biologist Helen Wallace, a spokesman for GeneWatch UK, said that while Dr Venter's team had managed a technical feat, it was some way from being artificial life.

'Venter is not God...He's a long way from creating life,' she said.

AGENCE FRANCE-PRESSE, NEW YORK TIMES