Burnley Road in Cliviger is closed between the railway bridge and Monarch garage for at least 10 days while investigations and repairs are carried out.
Motorists had a shock last week when the sinkhole appeared virtually overnight.
Barriers were erected around the hole, which has been filled in as a temporary measure and traffic lights installed to allow investigations into the cause of the sink hole to take place.
A spokesman for Lancashire County Council said: "We inspected what was originally thought to be a pothole on Burnley Road in mid-July, however, before we had chance to repair it we had a call to let us know that it had developed into a sinkhole.
"We immediately put up barriers and have now carried out further investigation. It appears to have been caused by a collapse to a culvert.”