Mystery surrounds BBC Radio 4 going off the air this morning as an emergency alarm went off in the studios.

Listeners to Radio 4's flagship Today programme heard presenters Nick Robinson and Martha Kearney trying to talk over an alarm loudly shrieking "please leave the building immediately".

Robinson told listeners they would go to a pre-recorded report "while we find out what is going on".

But after the four-minute report had ended, only silence was heard. The silence continued for six minutes before another pre-record began and ran up until 7.55 am when the breathless presenters returned to the airwaves after running back from the fire assembly point in the BBC car park outside the London studios.

Robinson announced: "Alarm over. There wasn't an emergency but we have to follow procedures".

Radio 1 and Radio 2 were also forced off the air by the mystery alarm.