Metsavas, 38, a former Estonian Defence Forces (EDF) officer received a 15 year- and six months prison sentence, and Volin a six-year sentence.

The sentence is dated from when the pair were first taken into custody, in September 2018.

Metsavas and Volin were found guilty of aiding Russian military intelligence agency, the GRU, in non-violent activity against Estonia in collecting and forwarding Estonian state secrets, classified external information and internal information to the GRU.

Most of the compromised information comprised state secrets, as well as internal information. The duo's activities predominantly took place in Estonia and Russia. Communications channels, as well as on-the-spot meetings, were used to hand over the information, it is reported. Remuneration for passing on the information was reportedly below €20,000 in Metsavas' case.

Metsavas carried his activities over a period of more than 10 years, while Volin's activity lasted for approximately half of that time, with a significantly smaller role. In addition to the aforementioned crimes, Volin was also convicted of two crimes involving the illegal handling of ammunition.

Public Prosecutor Inna Ombler, leading the investigation, said that the handling of offenses against the state is a prosecutor's office priority .

"When it comes to crimes against the state, the collection and handling of evidence is very specific. The evidence collected in the criminal case in question contained Estonian state secrets and classified external information, which is why it was wise to settle the criminal case in a compromise procedure," Ms Ombler said.

Director general of the Internal Security Service (ISS) Arnold Sinisalu said that the state must be able to prevent and hinder intelligence used against Estonia, as well as to prove the guilt of traitors on apprehending them.

"Let the incident in question along, with previous ones, be a clear message to those in doubt ‒ when attracting the interest of the Russian special services, it is more sensible to talk to us about it yourselves," Mr Sinisalu said.

"Clearly, opponents are anxious to conceal the recruitment of secret associates from everyone concerned, but we will find them sooner or later," he added.

The case was discussed at a closed hearing. Additionally, Metsavas was ordered to pay €1,350 in costs, while Volin was similarly ordered to pay €2,532 for compensation levies, expert analysis fees and lawyer fees.

The court's judgment has not entered into force at the time of writing.

Six apprehended for treason in last decade

Since 2008, the ISS has apprehended six persons engaged in acts of treason, including Metsavas and Volin, and 11 persons who carried out a crime against the state of Estonia to the benefit of Russian special services. All have been prosecuted under the direction of the prosecutor's office and have been convicted by the courts.

Metsavas had appeared on a TV talk show in 2016, stating that there were many ethnic Russians serving in the EDF (of which he was one) who were Estonian patriots, a tag he applied to himself.

In another high profile case, Herman Simm was convicted of treason in 2009 and sentenced to twelve and a half years in prison, for passing on information to the Russian Foreign Intelligence Service (SVR) over a period of nearly 13 years. In 2018 Simm, then 71, appealed for early release, but the appeal was rejected.

In September 2014, Estonian intelligence agent Eston Kohver was abducted by Russian agents under disputed circumstances close to the Estonian-Russian border. He was later sentenced in a Russian court to 15 years in prison for alleged espionage. Two years later, in September 2016, Kohver was exchanged in a ''spy swap'' at a border checkpoint, for Aleksei Dressen, who had been sentenced in to 16 years in prison for relaying sensitive information to Russia in 2012.

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