The British Army has tested out its capability of moving tanks through the Channel Tunnel to prepare for an invasion of Eastern Europe, say reports. A secret exercise carried out on Tuesday 17 January moved five Challenger tanks onto trains and through the tunnel to France and back.

The exercise was reportedly to test the rail network in case a UK military presence was needed in Eastern Europe. The Ministry of Defence (MoD) sent a tweet around midnight confirming an operation had taken place.

The operation comes days after Poland welcomed thousands of US troops, along with armoured vehicles and tanks, on Thursday 12 January, in the biggest deployment of US troops in Europe since the Cold War.

The mobilisation, which the Kremlin labelled a "threat", was part of Operation Atlantic Resolve, aimed at deterring an ever-aggressive Russia from Nato's Eastern border.

Then on Monday around 300 personnel from the US Marines Corps landed in Norway on a training mission around 1,000 miles from the Russian border. The marines will take part in Joint Viking exercises alongside British troops in March.

The MoD told Sky News this latest exercise had tested "the viability of using the Channel Tunnel to move vehicles and equipment to mainland Europe, adding to the existing range of options available and increasing the agility of our Armed Forces".

Sky News say that they have spoken to defence sources who affirmed that the exercise had been planned for a long time and the Channel Tunnel rail line could be commandeered by the British army in the event of a crisis.

When moving around the UK, the Army has their own railway line that joins the national rail network. But the military uses a private contractor, Leidos, to move equipment through the Channel Tunnel.

In October, Defence Secretary Michael Fallon announced that Great Britain will send tanks, drones, and 800 troops to Estonia in early 2017 to bolster Nato forces amid increasing tensions with Russia. The British armed forces will be joined by troops from France and Denmark.

Britain leads the bloc's Very High Readiness Joint Task Force, which is on standby to react to an attack on a Nato country within 48 hours.