BOSTON — The utility company involved in last month’s explosions and fires in three Massachusetts communities have been ordered by state regulators to halt all non-emergency natural gas work. The moratorium imposed Friday by the Department of Public Utilities covers all parts of the state serviced by Columbia Gas. While officials say the order won’t interrupt the ongoing restoration of natural gas to customers in Lawrence, Andover and North Andover, any other work must be approved in advance by the agency. A agency spokesman says the directive follows a preliminary report issued Thursday by the National Transportation Safety Board on the Sept. 13 deadly disaster. Columbia Gas said it won’t comment until the investigation is complete.The report found over-pressurization during a pipeline replacement project led to the explosions and fires that killed one and injured 25 others.