The news came out in Belgian newspaper De Morgen on Thursday (14 February), which cited ADIV sources.

A more senior ADIV officer, Clement Vandenborre, the head of its counter-intelligence department, has also been barred from his office after being accused of illegally shredding sensitive documents and of broader mismanagement, De Morgen added.

If the Belgian major is proved to have worked for Russia, the case would have echoes of Herman Simm, an Estonian defence ministry official, who was convicted of Russian espionage in Tallinn in 2009.

The ADIV's main task was to protect military secrets in Belgium, which is a Nato member, and overseas.

Brussels is also home to EU institutions and the Nato headquarters.

These are meant to be protected by their own internal security services and by Belgium's civilian intelligence service, the VSSE.

But a Russian penetration of the ADIV would pose a threat to everybody due to the interlinked nature of Nato and EU member states' systems, as the Estonia case showed 10 years ago, when Simm leaked 386 classified EU and Nato-linked files.

UK poisoning

The Belgian revelation came the same day a third Russian spy was named in a UK assassination plot.

Denis Sergeev, a senior officer in Russia's GRU military intelligence service, also took part in the attempted murder of Sergei Skripal, a former Russian spy, in Britain last year, Bellingcat, an investigative website, reported.

The Skripal case, and others, showed that "Russian military intelligence breaks the established 'red lines' even in peacetime and conducts aggressive and cynical operations against the West," Lithuania's state security service, the VSD, said in its "threat assessment" for 2019 earlier this month.

"Russia invokes Cold War-like foreign espionage methods that fell into oblivion in the West - influence operations, assassinations, and coup d'etats," the VSD said.

The Belgian spy affair comes amid long-standing security concerns in Brussels.

Spy capital

There were "about 250 Chinese and 200 Russian spies in the European capital," the EU foreign service said in a recent internal memo seen by German newspaper Welt am Sonntag.

"Brussels has now overtaken Vienna and there is now a greater density of so-called intelligence services from outside the EU in Brussels than here," Peter Gridling, the head of Austria's domestic intelligence agency, the BVT, said last year.

The level of Russian aggression in Europe is higher now than 10 years ago, Lithuania's report noted.

But even back in 2009, Alain Winants, the then head of Belgium's civilian security service, the VSSE, told EUobserver: "In Belgium, espionage, Russian espionage and from other countries, like the Chinese, but also others, [is] at the same level as the Cold War".

"For an intelligence officer, for a spy, this [Brussels] is a kindergarten. It's the place to be," he said.

Pretty trainee

"The threat of espionage is increasing day by day. A number of countries ... are continuing to seek sensitive and classified information," an internal EU commission memo, obtained by German newspaper Frankfurter Allgemeine Zeitung, also said in 2009.

"It [the foreign spy] could be the pretty trainee with the long legs and the blonde hair," a commission spokeswoman told press at the time.

The espionage alerts come amid wider Russian efforts to seek influence in the EU capital.

Vladimir Yakunin, a Kremlin insider under US sanctions, has been invited to speak at a conference in Brussels on 27 February.

Putin confidant

Yakunin used to be Russia's railway chief and a close confidant of Russian leader Vladimir Putin.

His Belgium trip was organised by an offshoot of the VUB, a university in Brussels, called the European Leadership in Cultural, Science and Innovation Diplomacy (EL-CSID)

The EL-CSID event, which will see him mingle with senior EU diplomats, was co-funded by EU institutions, according to Buzzfeed, a US news website.

The US sanctioned Yakunin for his support of Russia's invasion of Ukraine in 2014.

The EL-CSID symposium was entitled "Against the nationalist tide: A role for EU cultural and science diplomacy".

But Yakunin, in the past, also spoke at an event in Berlin that included guests from Germany's neo-Nazi NPD party its far-right AfD party.

This story was amended. The original version had incorrectly said Clement Vandenborre had been accused of working with Russia