The government strongly advised Britons yesterday not to travel to Hong Kong and Guangdong province, in southern China, as the mystery illness that has so far killed 78 people and infected 2,223 continued to spread panic through south-east Asia.
The Department of Health said it might add other countries to the list if evidence emerged of local epidemics.
All travellers bound for the Vietnamese capital Hanoi, Singapore, Toronto, Beijing and Shanxi in China, and Taiwan were warned to be aware of the dangers of severe acute respiratory syndrome (Sars), the deadly virus whose rapid spread around the globe through air travel has pro voked unprecedented health alerts from the World Health Organisation.
Last night, it emerged that a British journalist working for ITV, who arrived in Brazil from Asia, had been placed under observation in a Sao Paulo hospital after displaying symptoms consistent with Sars.
If confirmed, it would be the first case of the illness in Brazil.
The 42-year-old woman arrived in Sao Paulo from London, after spending time in Malaysia and passing through Singapore, to cover this weekend's Brazilian grand prix.
"She's in a hospital undergoing routine checks and we're awaiting results," said Gerard Lane, the editor in charge of the ITV team covering formula one, who declined to give her name. "We're not panicking."
Until the past few days, officials at the WHO believed that the virus was most dangerous through close contact with infected people, and that it was spread through coughing or sneezing, or through bodily fluids infecting medical staff.
They now concede, however, that it may also spread through shared building systems, such as water or sewerage, because a third of more than 700 cases in Hong Kong have occurred in one block of flats.
David Heymann, the WHO's head of communicable diseases, said: "We have found transmission not just through close contact. There is something in the environment."
Dr Heymann said the WHO had spoken to the International Air Transport Associa tion, which understood the public health reasons for the advice.
"This is the first time we have recommended people avoid an area," he said. "This is, of course, because we do not understand the disease completely, because there is no vaccine and there is no drug."
But he suggested that many countries seemed to be winning their battle against Sars. There had been no reports of further infection in Vietnam for a week and Singapore, Taiwan and Canada seemed well on the way to containing their outbreaks.
Travellers to the new areas identified by the British government as potential danger spots were advised to make themselves aware of symptoms, which include coughing and shortness of breath or breathing difficulties, and to be prepared for health screening before they were allowed to make return flights.
Authorities who find anyone with Sars-like symptoms will be told to delay their flights. Airlines have been given action plans in case travellers fall ill in transit and some airlines have already begun cutting flights to affected countries.
Travellers returning from these countries who show symptoms within 10 days are also being advised to contact their doctor.
Three cases have been reported in the UK so far.
The tough steps follow emergency measures being adopted across Asia, including the creation of quarantine camps, and for weeks residents in some countries have been wearing surgical masks to try to reduce the risk of infection. Scientists worldwide are trying to establish the cause of Sars, which is proving fatal in nearly 4% of cases.
The favourite culprit seems to be the corona virus, a common cause of colds, although another virus, the paramyxovirus, from the family that causes mumps and measles, may also be implicated.
Until the cause is known, no firm treatment can be found, although antibiotics and antiviral drugs are being used on patients.
The first outbreak seems to have started in Guangdong in November, but it was the middle of last month before worldwide worries surfaced.
