Screening passengers for flu or Sars when they arrive at UK airports would pick up only a relatively small number of cases, experts say.
During the Sars outbreak, visitors to Singapore were screened
The measure is being proposed as part of government plans in the event of a flu or Sars epidemic.
Health Protection Agency researchers say there would be checks before people flew, and few would develop detectable symptoms on even the longest flights.
The paper is published in the British Medical Journal.
Experts are focussing on air travel because it would be one of the primary routes for Sars of flu epidemics - which are most likely to emerge in South East Asia - to spread to the UK.
The HPA researchers considered the incubation periods for influenza and Sars and estimated the proportion of passengers with latent infection who would develop symptoms during any flight to the UK.
They found that the incubation period for Sars was too long to allow more than around 3% developing symptoms on flights from Europe, and 21% on the longest flights from east Asia.
Flu has a much shorter incubation period than Sars.
But the experts estimated that the average predicted proportion of people infected with flu and progressing during any flight would still be less than 10%.
They therefore said screening passengers from the Far East and Australasia derives the most benefit. But relatively few cases would be picked up even then, they said.
The team say having a policy where all passengers exposed to a person with flu or Sars could increase the benefit of entry screening, the sensitivity of entry checks would still be very low.
Writing in the BMJ, the researchers led by Dr Richard Pitman, said: "Entry screening is unlikely to be effective in preventing the importation of either Sars or influenza.
"The incubation period for Sars is too long to allow more than a small proportion of infected individuals to progress from symptomatic disease during a flight to the UK from any destination."
They added: "The proportion of individuals infected with influenza that is potentially detectable by screening is larger but still small, and most would be missed."
A spokeswoman for the Department of Health said: "We are fully aware of this modelling study and have taken it into account in our plans.
"Surveillance at ports of entry will help to identify those people who are unwell and need to be treated quickly.
"However, this measure is only one aspect of our pandemic plans.
"Antiviral drugs and vaccines will also play a very important role in reducing the impact of a pandemic.
"That is why we have purchased 14.6 million courses of anti-viral treatment which could help to reduce the severity of the illness and slow the spread of infection."