LONDON (Reuters) - The British police closed off a major motorway in northern England on Saturday after an explosion was heard near Britain's largest army training barracks. North Yorkshire Police said the report of the explosion was made at 3am GMT in the Catterick area, where more than 13,000 army personnel and their families are based.

There were no reports of any injuries, and the police said they were still investigating the possible cause. "We don't know what it was, it was a report of somebody hearing an explosion," a police spokesman said. "At the moment, there's nothing obvious, it was reported at night in the darkness. We're searching to try to find out exactly what it was that was heard.

"Obviously because it's in that area we've got to take all precautions, because of the location. The barracks are right next to the A1 (road)."

Police said their search was focused on an area outside the barracks and next to the A1, although they had not ruled out the possibility an explosion had occurred within the boundary of the barracks. "The safety of members of the public, including passing motorists, is our prime concern and therefore all security precautions must be taken to protect members of the public until we find out exactly what has occurred," the spokesman said.

Britain has been on its second-highest terrorism threat level of severe, meaning an attack is considered highly likely, since August.

Since then dozens of terrorism suspects, including some accused of plotting to kill police officers or soldiers and others charged with planning to behead a member of the public, have been arrested.

On Monday, Home Secretary Theresa May said Britain was facing the biggest terrorism threat in its history.