A vaccine against the deadly toxin ricin is safe and effective, a US study has shown.
Kamel Bourgass was convicted of plotting to use ricin in the UK
Ricin, extracted from castor oil beans, is one of the weapons which could be used in a bio-terror attack.
US research, published in Proceedings of the National Academy of Sciences, suggested the vaccine produced no significant side effects.
Scientists must now assess the ideal dosage and formulation of the vaccine before proceeding.
Ricin acts by damaging the organs, and a combination of pulmonary, liver, renal and immunological failure can lead to death.
It can be administered in food and water, or sprayed as an aerosol.
There is no antidote after the first few hours of exposure and, because symptoms do not appear until later and often mimic other illnesses, individuals often do not know if they have been exposed until it is too late for treatment.
The poison was used during the Cold War to kill Bulgarian dissident Georgi Markov in London. A specially-equipped umbrella was used to inject a pellet coated with 450 micrograms of ricin into his leg in an infamous attack on Waterloo Bridge in 1978.
Last year an Algerian, Kamel Bourgass, was convicted of plotting to use poisons - including ricin - to cause disruption, fear or injury.
He was arrested after anti-terror squad officers found a suspected chemical weapons laboratory in a north London flat in January 2003, although none of the toxin was discovered.
Researchers from the University of Texas Southwestern Medical Center carried out a study on 15 healthy people.
They were divided into three groups, who all received a series of three injections of various doses of the vaccine, called RiVax, which includes a genetically modified subunit of the toxin.
This acts to prime the immune system so it would recognise and attack ricin, if it came across the real thing.
All five of the individuals in the group receiving the highest vaccine dose produced ricin-neutralising antibodies in their blood, indicating their immune systems had responded.
Four of five in the intermediate dose group produced antibodies, while one of five in the lowest dose group did so.
Study participants only reported mild side effects, the researchers said - such as a sore arm at the site of injections or mild headache, which might be seen with any jab.
The human-produced antibodies were then injected along with active ricin toxin into test mice, and the mice survived.
Based on the protection they observed in the mice, the researchers projected that a human given the vaccine could withstand a lethal dose of injected ricin - estimated to be as little as approximately 500 micrograms, an amount that would fit on the head of a pin.
Dr Ellen Vitetta, who led the study, said: "Our major concern in this trial was safety.
"We have taken a very deadly toxin and genetically engineered it to be safe and to induce protective immunity in humans."
She added: "Now we need to tinker with the vaccine dose and formulation to give the longest-lasting and most robust immunity."
The work will include looking at whether the vaccine should be combined with an adjuvant - which can lengthen the time a vaccine is effective.
Professor Michael Lord of the Toxin Research Group at the University of Warwick said: "Both the US and UK governments have classified ricin as a potential reagent for bioterrorism.
"The threat is considered real particularly since Ricinus communis seeds (which produce ricin in abundance) grow over a wide range of warm climes, and the isolation of ricin from the seeds is relatively simple.
"It is anticipated that ricin would probably be used in the form of an aerosol, most likely in a confined space.
"At present there is no approved vaccine for ricin. Because of this, the Texas work is both interesting and potentially important."
The work to develop a vaccine grew out of the team's research into cancer treatment.
Ricin is being investigated to see if it might form the basis of cancer treatments because it can kill cells easily.