The three German nationals, named only as Herwig F., Ina F. and Thomas R., were accused of "having worked for a Chinese intelligence service", said the federal prosecutor's office.

From 2017, Thomas R. had "acted as an agent for an employee of the Chinese intelligence service MSS based in China", prosecutors said in a statement.

The suspect is said to have made contact with married couple Herwig F. and Ina F., who ran a company in the western city of Duesseldorf.

Together they are accused of obtaining "information on innovative technologies that could be used for military purposes".

To this end, the couple used their company to make contact with other businesses and scientific institutions, prosecutors said.

Between February 2017 and April 2024, "they repeatedly collected information that could in particular be useful for expanding China's naval combat power".

The material gathered included information on "boat engines, sonar systems, aircraft protection systems, motors for armoured vehicles and drones that can be used for military purposes".

The information was then said to have been passed back to China via Thomas R.

In one case, the couple are said to have signed an agreement with a German university to provide a "knowledge transfer" to China.

The first phase of this agreement was the preparation of a study for a Chinese "contractual partner" on state-of-the-art ship engine technology.

Behind the Chinese partner organisation was the MSS employee and the project was financed by Chinese state authorities, prosecutors said.

The three were also accused of purchasing three specialised lasers in Germany, which they exported to China without approval.

The lasers were controlled "dual-use" goods that have both civilian and military purposes, prosecutors said.

The group were arrested in April last year and officially charged in December.

Thomas R. was being in held in pre-trial detention, while the couple had been released in October following the suspension of their arrest warrant.

Germany has been unsettled in recent years by a number of high-profile cases of alleged spying for foreign powers, such as China and Russia.

A Chinese woman was arrested in October in Germany accused of spying on defence industry installations.

The woman allegedly reported to another suspected Chinese spy, Jian G., now under arrest, who worked in the office of a far-right German deputy in the European Parliament.

Police also launched a probe in December against a Chinese man who was reported to have taken photographs at naval bases on Germany's Baltic Sea coast.