Health protection officials are testing more than 30 healthcare workers after confirming a patient from Qatar is in isolation at a south London hospital with a new Sars-like virus.
The man flew into London from Doha for private treatment last month on a chartered aircraft.
Officials confirmed a Saudi patient with the same virus died in a private London hospital in August.
A further case of infection with the previously unseen virus has been confirmed in Saudi Arabia and at least one other case is under investigation.
The Qatari in hospital in London had recently travelled to Saudi Arabia.
World Health Organisation advice on travel remains unchanged, but officials are concerned ahead of the annual Haj pilgrimage to Mecca in Saudi Arabia next month. The Haj attracts up to one million Muslim pilgrims.
The new respiratory virus has been identified as a coronavirus, similar to the Sars virus which caused a serious outbreak in 2003.
Sars led to 750 deaths and paralysed travel across Southeast Asia.
A senior official at the UK Heath Protection Agency said: “There is no specific action for the public or returning travellers to take. We will share any further advice as soon as more information becomes available.”