The defendants are a married couple identified only as Herwig F. and Ina F., aged 73 and 69, and another man, 60-year-old Thomas R., on trial in the western city of Duesseldorf.

The three were arrested in April of last year. The couple were provisionally released in October, while Thomas R. remains in custody.

Prosecutors say they collected information on technology including boat engines, sonar systems, propulsion systems for armoured vehicles and military-grade drones.

They allegedly gathered the intelligence from February 2017 until their arrest.

Prosecutors accuse the three of obtaining the information for the Chinese intelligence agency MSS, with a special focus on intelligence China deemed useful to build up its maritime combat power.

The trio allegedly used the couple's company to build contacts with business enterprises and scientific institutions.

The couple also entered a cooperation agreement with a German university on a scientific transfer programme with a study focused on components used in, among other things, ship engines.

On behalf of the Chinese intelligence agency, the three defendants also procured three special lasers from Germany and exported them to China in breach of an EU ban on exporting dual-use systems.

In addition to working for a foreign intelligence agency, the trio is also accused of violations of the Foreign Trade Act.