LONDON (Reuters) - British military chiefs unlawfully killed a serviceman 51 years ago during secret experiments into the effect of deadly nerve agents, an inquest ruled on Monday.

Ronald Maddison, a 20-year-old Royal Air Force engineer, died in May 1953 following tests conducted by Ministry of Defense (MoD) scientists at Britain's Porton Down chemical and biological weapons laboratory.

An inquest held in 1953 was conducted in secret for reasons of "national security" and recorded a verdict of misadventure, concluding Maddison had choked to death.

But London's High Court overturned the decision in 2002 and ordered a new inquiry into claims Maddison was one of a number of human guinea pigs who believed they were simply taking part in tests to find a cure for the common cold.

The Ministry of Defense (MoD) said on Monday it would be taking legal advice following the ruling.

"The MoD notes the jury's findings and will now take some time to reflect on them," a spokesman for the MoD said.

"We will be seeking legal advice on whether we wish to consider a judicial review of the coroner's rulings and/or his direction to the jury," he added.

Maddison collapsed shortly after scientists placed a 200 mg patch of deadly chemical warfare agent Sarin on his arm.

Summing up the case, Wiltshire coroner David Masters told the jury the verdict depended on whether Maddison consented to the application of the nerve gas.

"If he did not give his consent, then a verdict of unlawful killing remains open to you," he said.

The coroner had told the jury to select one of three verdicts, either death by unlawful killing, death by misadventure or an open verdict.

Maddison's case is the only one in which a serviceman is believed to have died immediately as a direct result of tests, but other test veterans claim to have suffered long-term health problems.

The second inquest began in May this year in Wiltshire, western England, where Porton Down is located.

During the trial, documents revealed that the nerve gas tests had continued at Porton Down despite the death of the 20-year-old.

The government ordered a halt to the experiments but records showed sarin tests continued on three separate occasions later that year.

The history of nerve gases began in Germany in 1936 when Dr Gerhard Schrader discovered the gas tabun while working on insecticides.

Sarin was developed in 1938, a volatile liquid designed to paralyze and kill.