Jurors in the inquest into the death of a young North-East airman who died in secret nerve gas tests 51 years ago have been told they must reach one of three verdicts.

Ronald Maddison died aged 20 after being exposed to sarin in 1953 at the Porton Down base on Salisbury Plain, Wiltshire.

Leading Aircraftsman Maddison from County Durham collapsed and died within minutes of having 200mg of sarin drops put on to his arm in a chamber at the top secret base.

Summing up the case Wiltshire coroner David Masters told the jury that it must reach a verdict of either death by unlawful killing, death by misadventure or reach an open verdict.

He said: ''Your task will be to consider all the relevant facts and figures.

''You must ask yourself if Ronald Maddison consented to the application of a chemical warfare nerve agent in a non-therapeutic experiment. ''If he did not give his consent then a verdict of unlawful killing remains open to you.''

He told the jurors that they could reach a verdict of death by misadventure if they found that the experiment had unexpectedly gone wrong.

He said: ''A verdict of misadventure occurs if it is probable that the cause of death arose from some deliberate and lawful human act which has unexpectedly and unintentionally taken a turn that leads to death.''

Mr Masters told the Trowbridge hearing that an open verdict could be reached if ''the evidence does not disclose the means whereby the cause of death arose''.

The original inquest was held behind closed doors and ''for reasons of national security'' - a decision made at the highest levels of government in the 1950s.

But the long-running inquest has been reopened after years of campaigning against its open verdict.

Lawyers say the outcome of the inquest could lead to legal action by the veterans of Porton Down who claimed they were duped into taking part in similar dangerous trials.

The case summation is expected to last for five days.