Professor Pat Troop, the chief executive of the Health Protection Agency, warned that a pandemic would impose severe pressures on the NHS were it to strike the UK.
Costs would "hugely multiply" and careful planning was essential, she said.
Professor Troop was speaking as the Government confirmed that stockpiles of anti-viral bird flu treatment courses should reach 2.5 million next week. The Government ordered 14.6m courses of Tamiflu in March.
Yesterday, leading supermarkets said they were looking at how to source extra supplies of beef and lamb in case customers no longer wanted turkey at Christmas because of fears about bird flu. Sainsbury's said that sales of poultry and eggs were still "robust" and consumer confidence remained high while Tesco and Asda said that sales had dipped earlier in the week but had recovered.
A spokeswoman for Sainsbury's said: "Should people really go off turkey and not want it on Christmas Day, we are seeing what other meat people might want. We don't think that is likely because consumer confidence is still good."
Professor Troop said that the experience of Canada during the Sars outbreak of 2003 showed the potential impact. The cost to health services increased by 60 per cent during the outbreak in Ontario. "That gives an indication of the kind of cost you might have from a pandemic," Professor Troop said.
It was difficult to calculate exactly what impact it would have on the NHS because the pandemic strain could be mild and people would recover quickly, or it might be more severe and raise health costs.
Professor Troop was speaking at the launch of a report, Understanding the Burden of Disease, that estimates the total cost of treating infectious disease in England at £6bn a year. Of this, GPs had the biggest burden of costs at £3.5bn, while hospital admissions cost £900m.Reuse content