A RUSSIAN spy plane spent hours observing military barracks in the UK as troops below were forced to hide sensitive equipment.
Russian military chiefs exploited an international loophole to fly the spy plane over the UK.
People close to the Russian President, Vladimir Putin, said he used a “little known” loophole to spy on the UK.
The plane was a Antonov An-30B, an aerial cartography aircraft used for reconnaissance and transport.
A source said: “The Russians will never miss a trick.
"Some follow the spirit of the rules a little different from others.”
Countries are allowed a set number of flights over other states under the Open Skies Treaty.
The Open Skies Treaty is designed to give states a direct role in gathering information about military forces and activities and encourage trust between old military enemies.
Under the treaty, the UK accepted 12 flights this year and allocated three to Russia.
A 1995 declaration meant, however, that Russia could fly over the UK for a fourth time because the UK failed to give out its full allocation.
The Ministry of Defence said it was completely routine with “UK military personnel on board the jet”.
It comes after confirmation Russia has carried out more than a dozen bombing raids and nine cruise missile strikes against ISIS militants in eastern Syria.
Over the past three days, Russia has launched 18 bombing raids in the region as well as cruise missile strikes from submarines, the Russian Defence Ministry claimed.
The ministry said the strikes had helped to support attack raids by Syrian troops.
Meanwhile, the Syrian government, which is backed by Russia, regained complete control of the eastern city of Deir al-Zor on Friday.
The progress represented a huge blow to the jihadists as their last strongholds in Syria crumbles.
The latest military offensives have reduced the so-called ISIS caliphate to a pair of border towns at the Iraq-Syria frontier.
Deir al-Zor, on the west bank of the Euphrates River, is the largest and most important city in eastern Syria and is the centre of the country's oil production.
A military source said: ”The armed forces, in cooperation with allied forces, liberated the city of Deir al-Zor completely from the clutches of the Daesh terrorist organisation.”
The army, backed by Russian bombers, Iranian soldiers and Shi'ite militias, is advancing towards the last significant town held by ISIS in Syria, Albu Kamal, which is also located on the western bank of the Euphrates.
A rival offensive by Kurdish and Arab militias supported by a US-led coalition with air strikes and special forces is pressing ISIS fighters on the eastern bank.