The government can challenge an inquest verdict on the death of a serviceman who underwent nerve gas tests in 1953, the High Court has ruled.
An inquest ruled Ronald Maddison was unlawfully killed
RAF serviceman Ronald Maddison, from County Durham, died aged 20 after having sarin dabbed on his arm at the Porton Down facility in Wiltshire.
A coroner's jury returned a verdict of unlawful killing last November.
Mr Justice Collins said the case raised points "...which it would be impossible to say are not arguable".
"Of course, that is not to say the claimant (MoD) will succeed," he added.
The MoD argued coroner David Masters was "wrong in law" to leave a verdict of "unlawful killing by reason of unlawful act manslaughter" open to the jury.
The permission to seek judicial review will not take effect until 20 May, or until Mr Maddison's family has had a chance to decide whether or not to oppose the decision, the judge said.
He told the family's counsel: "Really, you are not going to be likely to persuade any judge the point is not arguable."
Ian Burnett, for the coroner, said Mr Masters also accepted there was an arguable case, especially as the legal points raised by the case were "not straightforward, and complicated by the time gap".
The original inquest - held in secret in 1953 - ruled Leading Aircraftman Maddison's death was caused by misadventure.
In 2002, the High Court quashed the verdict and ordered a new inquest should be held.
The second inquiry was prompted after ex-serviceman Gordon Bell complained to Wiltshire Police that he had been duped into similar tests.
The constabulary launched Operation Antler, which looked at experiments using chemical and biological agents at Porton Down government research centre between 1939 and 1989.
The operation found the coroner at the original inquest was "not apparently provided with all the potentially available material".