Europe's biggest oil refinery has resumed operations after most of its electricity supply was restored following a devastating fire more than three weeks ago, Shell said on Thursday.

"Repairs to the electricity supply at Shell Pernis have mainly been completed," the Anglo-Dutch oil giant said in a statement, adding "most of the factories in the plant have been restarted including those that needed permission from authorities."

The massive Rotterdam-based plant halted production after a huge blaze on July 29, which Shell said was caused by a short-circuit followed by a fire at a substation.

"We are investigating the underlying cause of the short-circuit," Shell said.

Firefighters brought the blaze under control in the early hours of that morning.

But a second incident occurred the following day during the cleaning of one of the factories when there was a leak of a colourless, highly reactive gas, hydrogen fluoride.

This incident too was being probed.

The refinery covers an area equivalent to 1,000 football pitches, and its pipework, if laid end to end, would be long enough to circle the Earth four times.

It has 60 factories on-site, making it also one of the largest refineries in the world.

The facility, based in the port of Rotterdam, can process more than 400,000 barrels of petroleum products a day.

source: AFP