More than 5,000 workers had to be evacuated after a power cut blacked out the BAE Systems site at Warton near Preston.
The plant was brought to a halt at around 12.30pm on Monday and almost 2,000 other consumers were also cut off in the vicinity of the complex. Supplies were not fully restored until early evening.
Thousands of BAE staff on site were sent home early and nightshift workers were told not to go in.
A company spokesman said: “A safe, controlled evacuation of employees on our Warton site was carried out on Monday afternoon following an external power outage in the area.
“All measures were taken to ensure our staff remained safe and informed throughout the outage and the subsequent evacuation.
“Power was restored to the site on Monday night and operations resumed as normal on Tuesday morning.”
Officials from Electricty North West were on site almost immediately after the blackout.
A spokesperson said: “Damage to an underground high voltage cable, caused by construction work, caused the interruption to supplies.
“The fault affected 1,889 customers - houses and businesses - in the area of Harbour Lane.
“We managed to get supplies back on in a phased way and everyone was reconnected by 6.30pm.