And relatives of 14 UK DVT victims have launched a multi-million law suit against the airlines concerned.
The London researchers revealed that 40 times more people suffered deep vein thrombosis (DVT) after long-haul flights than previously thought.
But many of these clots are "symptomless" and do not go on to develop into larger and potentially fatal clots.
Wearing special compression stockings, given out by some airlines, has been found to reduce the risks.
But the results are bound to raise concern among passengers.
The latest study, published in The Lancet, has already provoked controversy with scientists disputing the high number of cases found by consultant vascular surgeon John Scurr and his team.
An international meeting of scientists, aviation experts and air authorities in March launched an investigation into the number of DVT deaths following long-haul flights.
Victims
About 10 people a year die from blood clots in the lungs, like Emma Christofferson, 28, who died after a 20-hour flight from Australia.
And relatives of 14 UK DVT victims have launched a multi-million law suit against the airlines concerned.
Mr Scurr, of the Middlesex and University Hospitals, London, said his team had used extremely sensitive ultrasonographic assessment which enabled them to pick up more cases.
His team studied over 200 passengers who had travelled on flights of eight hours or more.
Half were given the special stockings and showed no symptoms - 10% of the others did show clots in their calves.
Mr Scurr said:"What we are doing is picking up the small clots, some will just go away, but others like seeds will germinate and become bigger clots."
Mr Scurr said there was no-one who could not fly, even those who had previous problems.
He said following medical advice and using stockings should cut the risk of clots and that the airlines were working with scientists to cut risks.
Mr Scurr said: "The airlines have been working with us for some time now.
"They are very familiar with these results and are giving passengers advice.
"For the majority of people, though, there is no risk."
Further studies
In a commentary piece also in the Lancet, Dr Jack Hirsh from McMaster University in Ontario, Canada, said more detailed and extensive studies were needed before firm conclusions could be drawn.
"What is needed are rigorously designed and adequately powered studies to resolve the issue.
"It would be premature to legislate that airlines change the seating configuration or introduce other costly procedures until there is more information on the extent of the problem and on the effectiveness of much simpler preventative measures."
Roger Wiltshire, Secretary General of the British Air Transport Association told the Today programme "We accept there is a risk with immobility.
"That is why the airlines have done an awful lot in the past 12 months to improve the advice they give to passengers about exercise and sensible hydration in flight."
But he said it was not the responsibility of the airlines to provide compression stockings.
DVT sufferer John Hodges told the BBC that he had suffered problems on a flight back from the US.
He started to feel breathless and had to be taken to hospital.
Mr Hodges said that if he ever flies again he will ensure he will make sure he takes all precautions necessary to protect his health.
"If ever I fly again I would certainly acquire a pair of stockings.
"The medical services seem to have a consensus that they are good for you."