SOLDIERS believed they were under attack from terrorists after being rocked by an explosion near Britain's biggest Army base, it has emerged.

More details have emerged about what happened when troops on security duty at Marne Barracks, Catterick, issued an alert to their superior officers at about 3am on Sunday.

North Yorkshire's deputy chief constable Tim Madgwick said soldiers on duty reported hearing a huge blast and said they saw dazzling light outside the guardroom, which is yards from a slip road to the southbound A1.

Eight residents have since told police they also heard the explosion, the cause of which is baffling police.

In the aftermath of the blast, four houses on the garrison were evacuated and soldiers told police of their shock as explosives detection teams and the National Police Air Service were called in.

Counter-terrorism officers were also sent to the scene to examine whether extremists had targeted the garrison for the first time in its 100-year history.

A six-mile section of the A1 was closed for 14 hours, causing major tailbacks in the area, as it was established a suspicious vehicle had been sighted near the barracks - the home of 5 Regiment Royal Artillery and 1st Battalion Mercian Regiment - which had made off two hours before the blast.

But, despite an intense search of the area, detectives admit they have yet to establish what caused the security alert.

Mr Madgwick said: "One of the guards who had been outside at the time described it as a particularly loud explosion.

"He subsequently thought it might have been part of an attack and the guards inside the guardroom saw a flash across the windows.

"They immediately called the police and their Army colleagues."

Mr Madgwick said when police arrived at the scene it had been impossible to identify the cause of the explosion due to darkness, but officers rapidly established there was no obvious damage to the perimeter of the base or structures inside or outside it.

He said officers made a swift decision to close the A1 on the basis that they could not identify the damage or what the ongoing risk was to Army personnel, police staff or the travelling public using the A1.

When asked about concerns over whether terrorists had launched an attack, Mr Madgwick said: "We were aware of the national state of readiness and the risk status that we were all working under.

"We are aware that the armed military bases area a moderate risk, we are very conscious that Catterick is the largest military base in the country and is always going to be a subject of interest, so we had that as one hypothesis."

While North Yorkshire Police initially ruled out terrorism, Mr Madgwick said all other theories for the blast, such as it being caused by contractors working in the barracks or on the A1, a nearby quarry or a farm, had been checked and dismissed. However, the suspicious vehicle has been eliminated from the inquiry after officers traced and interviewed the driver.

Nothing was found apart from a box of ageing detonators near the barracks perimeter fence, which were destroyed in a controlled blast.

Mr Madgwick said: "It is frustrating for us as investigators. We still don't know what caused the explosion."

An Army spokesman said it had not launched an inquiry into the incident and that it remained happy with the security measures at the garrison.