A bird flu pandemic will hit Britain - but not necessarily this winter, the chief medical officer has said.
Sir Liam Donaldson said a deadly outbreak would come when a strain of bird flu mutated with human flu.
He told the BBC's Sunday AM show it would probably kill about 50,000 people in the UK, but the epicentre of any new strain was likely to be in East Asia.
The UK has so far stockpiled 2.5m doses of anti-viral drugs - and may restrict travel if there is an outbreak.
On Saturday, UK tests confirmed a case in Romania of a strain of bird flu which is potentially deadly to humans, sparking fears avian flu could spread to the UK through migrating birds.
A pandemic would occur if this strain of bird flu mutated with human flu - which spreads very easily - to create a new strain.
He said it was "less likely" that any new flu strain would come this year.
However he said that if the flu first emerged in another part of the world it would give UK scientists time to try to create an effective vaccine against the virus before it arrived in the UK.
"We can't make this pandemic go away, because it is a natural phenomenon, it will come," he said.
"But what we can do is to limit its impact."
He said a contingency plan was being released on Thursday, outlining the steps the government would take in the event of an outbreak.
If a new strain did hit the UK before a vaccine was created, Sir Liam said an extra 50,000 would probably die - and a death toll of 750,000 was "not impossible".
"In a normal winter flu year... flu actually kills in excess of 12,000 people," Sir Liam said.
"But if we had a pandemic, the problem would be that our existing vaccines don't work against it, we would have to develop a new vaccine, and people don't have natural immunity because it hasn't be around before."
The total death toll depended on whether the mutated strain was a mild or serious one, he said.
However, Dr Martin Wiselka, consultant in infectious diseases at Leicester Royal Infirmary, said a death toll of 50,000 was a "complete guess".
"It could be worse, it could be better. I think initially it could be worse than that," he said.
"When a new strain arrives it tends to be more virulent but then it slows down. But the honest answer is we don't know."
If a pandemic did materialise, the top priority other than vaccination would be anti-viral medicine which would "stop some people dying", Sir Liam said.
The UK has ordered 14.6m doses of anti-viral drug Tamiflu - enough for 25% of the population - which would alleviate symptoms among people affected.
Sir Liam admitted the UK only had 2.5m doses so far, with 800,000 new doses arriving every month.
The chief medical officer said key NHS workers would be the first to get treatment, but during any outbreak it would soon become apparent which age group was worst affected, and treatment would be targeted towards them.
Measures such as controlling movement of populations were not so important, because flu transmitted extremely quickly, he said.
However, the government might advise people to avoid non-essential travel in a bid to slow the spread of the virus, he acknowledged. It also might prove necessary to close schools and other public buildings.
But this would not affect air travel, he said.
Sir Liam said flu pandemics were things which came in "natural cycles" every 10 to 40 years, with the last taking place in 1968/69.
However he said that three decades ago there were no anti-viral drugs to combat the virus in its initial stages, as there are now.
He also said the situation was not comparable to the Spanish flu pandemic of 1918 which killed millions around the world, as we now lived in "different times" with great advances in hospitals and medical science.
"We have to get the [new] virus from wherever it occurs... and get it into our labs and then make a vaccine," he added.