Herts County Council has launched an urgent review to find out why a large number of street lights failed to come on at all on Thursday night.
The lights affected were among those which have controversially been adapted to switch themselves off from midnight to 6am to save cash and cut down on light pollution.
Instead many failed to come on at all, with the problem being identified in streets across the county.
A spokesman said today: “We were anticipating a small number of possible faults around the spring clock change at the end of March following a similar problem last year which was caused by a manufacturing fault in a photocell.
“A major programme of remedial works was carried out at no cost to the county council and extensive testing appeared to show the problem had been resolved.
“We are working with our then contractors to understand the latest problem and to take whatever action is necessary to put it right.
“Thanks to everyone who has reported faulty streetlights on our new highways fault reporting system.
“We hope to resolve this problem shortly but if any issues continue into next week please report these , or any other highways faults online. Click here to report a fault
Were you affected by the partial blackout? Let us know, email details to firstname.lastname@example.org