* Virus is different from any previously found in humans
* International authorities on alert, no evidence of spread
* Virus from same family as SARS and common cold
* Only two confirmed cases worldwide so far (Adds fresh comment, UK advice to health workers)
By Kate Kelland, Health and Science Correspondent
LONDON, Sept 24 (Reuters) - A Qatari man struck down with a previously unknown virus related to the deadly SARS infection and the common cold is critically ill in hospital in Britain, the World Health Organisation said on Monday.
The U.N. health body put out a global alert on Sunday saying a new virus had infected the 49-year-old man who had recently travelled to Saudi Arabia - where another man with an almost identical virus had already died.
Britain's Health Protection Agency (HPA) and respiratory disease experts said there was no immediate cause for concern, although authorities were watching out for any signs of the virus spreading.
"This new virus ... is different from any that have previously been identified in humans," the HPA said.
Any suggestions of a link between the virus and Saudi Arabia will cause particular concern in the build-up to next month's Muslim haj pilgrimage, when millions of people arrive in the kingdom from across the world, then return to their homes.
The virus, known as a coronavirus, comes from the same family as SARS which emerged in 2002 and killed 800 people.
"This is now an international issue," WHO spokesman Gregory Hartl said. "The (Qatari) patient is still alive but, as we understand, in critical condition," he said.
The HPA said it was recommending the Qatari patient be treated in isolation by doctors and nurses wearing gowns, gloves respirators, goggles and other protective equipment.
It did not recommend any specific actions for travellers or other members of the public.
The Qatari man first showed symptoms of an acute respiratory infection while he was in Qatar, the WHO said.
He spent some time in intensive case in Qatar and was later flown to the UK where he was currently in a London hospital's intensive care unit, being treated for acute respiratory infection and kidney failure. Officials declined to name the hospital.
Andrew Easton, a virologist at Britain's University of Warwick, said that with only two cases so far, it was difficult for experts to estimate the potential threat.
"The important thing is to be aware of the virus and to be on the lookout for any evidence that it is more than a rare chance event," he said.
Coronaviruses are a large family of viruses that includes causes of the common cold but can also include more severe illness such as the virus responsible for SARS.
SARS, or Severe Acute Respiratory Syndrome, appeared in China in 2002 and infected more than 8,000 people worldwide, killing around 800 of them before being brought under control.
John Oxford, a professor of Virology at Queen Mary, University of London, said he was not too concerned as the new virus was "more likely to join numerous other members of the coronavirus family and behave like a nasty infection rather than join the exception group like SARS".
"SARS was very quick off the mark infecting hospital staff," he said in an emailed comment. "And this new virus does not to me appear to be in the same 'big bang' group."
The WHO said it was in touch with health authorities in Britain, Qatar, Saudi Arabia and the Stockholm-based European Centre for Disease Prevention and Control (ECDC)
"We're asking for information from whoever might have seen such cases, but as of the moment we haven't had any more notifications of cases," said Hartl.
The HPA said it had conducted lab testing on Qatari case and found a 99.5 percent match to a virus that killed a 60-year-old Saudi national earlier this year. The Saudi man's virus was not identified as a new kind of infection at the time of his death.
There was no evidence of ongoing transmission, said the head of the HPA's respiratory diseases department, John Watson.
"In the light of the severity of the illness that has been identified in the two confirmed cases, immediate steps have been taken to ensure that people who have been in contact with the UK case have not been infected, and there is no evidence to suggest they have," he said. (Additional reporting by Angus McDowall in Riyadh; Editing by Andrew Heavens)