A study of transfusion patients given blood contaminated with the human form of mad cow disease has indicated the 24 still alive are at "substantial" risk.
Prof John Collinge's assessment follows his investigation into the third person infected from the original group of 66.
The patient, the first to be diagnosed with variant Creutzfeldt-Jakob disease (vCJD) while alive, has since died.
Two others died before their illness was confirmed. The other 39 died for reasons unrelated to the disease.
The name of the third patient to be infected has not been released, but it is known that he received the blood transfusion at the age of 23.
He later became ill, and seven and a half years later he was referred to the NHS National Prion Clinic at the National Hospital for Neurology and Neurosurgery where his symptoms were confirmed to be caused by vCJD.
He joined an experimental Medical Research Council trial for a treatment called Prion-1 which began in 2004, in which patients were given a drug called quinacrine, but he died a year later at the age of 32.
The diagnosis of vCJD was confirmed after his death after his tonsil tissue was examined.
Diseases in humans such as vCJD which are caused by prions, rogue proteins, are known to have long incubation periods, and it is possible that a person could be silently infected for more than 50 years before developing symptoms of the disease.
During this time such a carrier of infection poses a potential risk to others through blood transfusion and contamination of surgical and medical instruments.
The incubation period when prions pass from human to human is thought to be much shorter than when they pass from one species to another.
So prions that enter the body through a blood transfusion rather than through eating meat infected with BSE (bovine spongiform encephalopathy) are thought to cause vCJD to develop more quickly.
Experts believe that, based on the cases seen so far, infection from a blood transfusion can develop in just six or seven years.
Professor John Collinge of the MRC Prion Unit said: "That three individuals from this small group of people that we know to have been exposed through blood transfusion have already developed vCJD infection suggests that the infection may be efficiently passed by this route.
"So the risk to remaining individuals is likely to be substantial."
He told BBC Radio 4's Today programme: "We know about these particular instances because individuals went on to develop vCJD.
"But of course people who are silently incubating vCJD at the moment may be blood donors and there is no way of knowing where that blood is going."
He added: "'A national tonsil tissue screening study being performed by the Health Protection Agency may soon give estimates of the number of people who are silently infected with prions.
"This information is vital for public health planning given the relative ease with which prions seem to be passed on by blood transfusion."
He said it was possible people at high risk could have their tonsil tissue tested in a bid to diagnose if they are infected.
Professor Collinge added: "Although we do not yet have an effective treatment for any form of CJD, a reliable tonsil test could allow people with vCJD to access experimental treatments early."
Writing in the Lancet, Kumanan Wilson of Toronto General Hospital and Maura Ricketts of the Public Health Agency of Canada said: "This third case considerably strengthens the inference that transfusion transmission is possible and suggests that the causative prion can be efficiently transmitted via this route."
A spokeswoman for the National Blood Service said a large number of measures had been put in place to protect people receiving transfusions including withdrawing any blood products donated from a person who later develops vCJD.
She added: "Whilst no medical procedure, including blood transfusion, can be 100% safe, our prime concern is always the safety of patients through the quality of blood.
"To date there have been three cases of vCJD infection where blood transfusion is a possible source of infection. However, the National Blood Service issues nearly two million units of blood every year."