The health protection agency (HPA) admitted that it was concerned that the problem in Toronto had only just dawned on NHS staff here.
In dramatic proof of the international threat of Sars, airline passengers travelling to Britain will soon be shown official health videos and handed leaflets before they land. The agency's US equivalent, the centres for disease control, in Atlanta, Georgia, is soon to complete an inflight video for use by all big airlines.
More than 260 people out of 4,439 reported cases have now died, with four new countries, Saudi Arabia - where a 13-year-old boy has died - Bahrain, Bulgaria and New Zealand investigating their first suspect cases.
Hospitals in Beijing in China, and Taipei in Taiwan, were sealed off in an attempt to limit the spread of the disease. The 1,200-bed, 2,300-staff People's hospital in the Chinese capital was barricaded by guards refusing to let anyone in or out. Medical staff told Reuters that at least 60 staff had caught the virus working in a makeshift isolation ward without proper equipment.
It is two weeks since news that Britain's sixth and most recent probable Sars case was admitted to hospital and five days since he was allowed home. But the addition of Toronto in Canada to the danger list for would-be travellers and the post-Easter start of new terms at schools and universities has finally brought the threat home.
Doctors' leaders here warned the public not to panic but the Conservatives called on the government to introduce powers to detain travellers entering Britain with symptoms.
The chief medical officer, Professor Sir Liam Donaldson, and HPA officials were left to handle the flak.
They deny there is any need to screen passengers on arrival from the worst-hit parts of the world, and are relying on checks before boarding and watchfulness of aircrew while people are in transit.
However, Angus Nicholl, director of communicable disease control at the HPA, revealed concerns that people who had been to Toronto had not been appearing "on the NHS radar screen".
Official guidance only added the city to the travel history for which doctors should look out on Saturday, yet on Tuesday non-essential travel to the city was effectively ruled out by the World Health Organisation.
This was in contrast to recent returnees from Asia, or people who have been in close contact with them, whose condition has sparked extra vigilance for more than a month. Many suspect cases that turned out not to be Sars had been investigated, Professor Nicholl said.
"That is good. It suggests the system is working ... We are really a bit concerned we have not been seeing many people from Toronto that just call. People in the NHS haven't been looking for it."
At Heathrow, Judi Priestly, of Dunstable, Bedfordshire, clutching a face mask on her return from visiting her pregnant daughter in Hong Kong, said some people there were wearing masks all the time. "There is panic if you hear of a case nearby, but you have to batten down the hatches and make the best of it."
She had been tested with an ear thermometer as she left Hong Kong but had been given no medical advice and was not screened at Heathrow.
The Conservatives want the government to impose powers to detain people suspected of having Sars, forcing them to be examined or moved to hospital. Liam Fox, the party's health spokesman, said: "The government's response has been feeble and ministers have been all but invisible."
The Liberal Democrat health spokesman, Dr Evan Harris, criticised the amount of information available. "People need reassurance the government has made adequate contingency plans for any widespread outbreak of Sars in the UK".
But Professor Donaldson said the six British cases had been detected quickly and brought under control. "We remain very concerned about the situation worldwide. We are tracking this disease internationally. We are in very close daily contact with the World Health Organisation and other countries involved and far from being feeble or complacent, are working very hard."
Meanwhile, the health scare disrupted the education of more pupils, as Cheshire county council decided that 21 students and three teachers on a cultural exchange in Beijing will not be allowed to return to Knutsford high school for 10 days after they fly home.
Several schools, including Eton and Winchester, have refused to accept students for 10 days after their return to the UK, while the headteachers of Harrogate Ladies College and Wellington School in Somerset have gone into quarantine with pupils.
