Professor John Oxford, an expert in virology, said that even though only 229 cases of swine fu have been confirmed in the UK so far, he was certain of a flu pandemic which would take hold as autumn begins.
Children going back to school, universities re-opening and people returning to work after the summer break would give swine flu the "opportunity" it needs, he said.
He warned that a pandemic was likely to affect the UK before a vaccine becomes available, adding that the vaccine - which is expected to be ready in October or November - would only cover about 15 per cent of the population.
Prof Oxford, of Queen Mary School of Medicine, University of London, praised campaigns urging people to observe good hygiene - such as using tissues and not coming into contact with infected people - but said they would not halt the virus altogether.
"Problems at the moment are fairly low, with schoolchildren and the odd sporadic case in the community.
"But when children go back to school in September the virus has an opportunity, and normally it takes it.
"That's the scenario we should prepare for."
Prof Oxford said children were usually involved in an outbreak because they liked to be in close contact.
"Children are very social, they play together and put their arms around each other, they kiss each other. Also, their hygiene levels are not very high."
He added: "I view it [tackling flu] like a medieval castle, with several layers of defence: you've got the outer layer which is hygiene and social distancing, then the virus breaks through that and then we have antivirals like Tamiflu and then the final layer is the vaccine.
"You chuck all of this at the virus. You can't stop it, but you can slow it down."
Prof Oxford said there had already been sporadic cases which were not linked with anyone coming into the UK from the US or Mexico.
"That does suggest that the virus is silently spreading around," he said.
He said the current estimate that 50 per cent of the population may get infected was on the "low side".
He added: "The virus will be like a bushfire."
In order to avoid flu, people should keep their distance from others, wash their hands regularly and cough into the crook of your arm.
The World Health Organisation's current threat level of a flu pandemic is at level five. Declaration of a pandemic would move it to level six.
A spokesman for the Department of Health said: "Our strategy to slow the spread of the disease appears to have been effective, and the UK's arrangements continue to ensure that we are well-placed to deal with this new infection.
"But we must not be complacent - it is right to prepare for the possibility of a global pandemic."