Software engineer Simon Finch denies breaching Britain's Official Secrets Act by emailing details of the system, which a prosecutor described as critical to UK security, to undisclosed recipients in October 2018.

The 50-year-old, from Swansea in Wales, is also accused of refusing to give British authorities access codes to three electronic devices at his trial at London's Old Bailey court.

"The document he attached to the email set out technical detail classified as secret and top secret... (and) contained detail about the operational performance of the system," prosecutor Mark Heywood told jurors.

"Expert evaluation has concluded that the release of information of that kind, for example to a hostile adversary of the UK, would give them an understanding of the function of that relevant system which in turn would allow them methods of countering it."

Heywood said the disclosure could create a "risk to the UK and to others with whom that technology is shared".

The jury will hear more details about the system during closed sessions of the court. Journalists and the public will be barred from those proceedings.

Finch worked for private contractors including BAE Systems and QinetiQ from the late 1990s until 2018.

Armed with "expert technical ability", he was involved in the design, testing and configuration of the UK missile system still used by British armed forces, according to Heywood.

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BAE SYSTEMS

QINETIQ GROUP