Two new species of a variety of virus have been discovered that are so different from any previously known they may be a new life form, French scientists say.

Dubbed Pandoravirus because "opening" them has released so many questions about life, they are unlike anything ever seen before and are twice as large as any viruses previously discovered, researchers at the CNRS, the French national research agency, said.

More than 90 per cent of the Pandoravirus genes are new to science and have no known counterparts in other viruses, bacteria or higher forms of life such as animals, NewScientist.com reported Friday.

The genome of one of the new varieties, P. salinus, is twice the size of that belonging to the previous record-holder, Megavirus chilensis, the researchers said, and the Pandoraviruses are also larger than many bacteria, and even larger than some cells of plants and animals.

"No microorganism closely related to P. salinus has ever been sequenced," said CNRS researchers Jean-Michel Claverie and Chantal Abergel, who discovered the species.

"One of our jokes is that either they are from outer space, or from a cellular ancestor that's now disappeared," Abergel said.

The Pandoraviruses, like all viruses, can't copy or process their own DNA and must have a host to do that for them.

Pandoraviruses are not, strictly speaking, alive because they cannot make their own energy -- a central definition of life -- said virus expert Gary Foster of the University of Bristol in Britain.

"It fulfils all the criteria for being a virus, except the sheer size, and that's what's blowing people's minds away," he said.

He disagrees, however, they should be given their own taxonomic category as a new form of life.

"If you change the domains for every new weird thing, it would be an absolute mess," he said.

For now, Foster said, he thinks the viral definition is suitable.