A thick blanket of fog slowed roads - and flights - in Perth on Friday morning.

A thick blanket of fog slowed roads - and flights - in Perth on Friday morning. Photo: Aleisha Orr

A thick band of fog descended on Perth this morning, delaying flights and causing major congestion on the roads in what was described as a 'once in a year' event by the Bureau of Meteorology.

BOM issued a road weather warning at 3.38am on Friday morning, warning motorists to take car as the fog took hold.

A spokesman for Perth Airport confirmed the fog had caused delays to four flights - two flights in and two flights out of Perth. But a Qantas spokeswoman said the airline's flights had not been delayed.

The Swan River's iconic boatshed loomed from a misty cloud on Friday morning. Click for more photos

Perth covered in fog, Friday August 8, 2014

The Swan River's iconic boatshed loomed from a misty cloud on Friday morning.

Main Roads issued a fog warning to motorists at 6.17am, advising drivers to use their fog lights and to leave extra distance between cars.

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The Mitchell and Kwinana freeways were still moving slowly at 7.30am, with traffic slowed to a call on other major arterial roads including Leach Highway, Tonkin Highway and Orrong Road..

However the fog has been a hit on social media, with many fog shots circulating on news feeds.

BOM's Neil Bennett said that although fog wasn't unusual in Perth, Friday morning was an exception, describing it as a "once in a year" event.

"It's very unusual for fog to last this long in Perth. Usually it has dispersed by 6am or 7am," he said.

"But I suspect this one will stick around until at least 9am. We had rain last night and that has kept the moisture content of the air very high, and the overnight low of 12.9 degrees being lower than the dew point, which was 13.5 degrees this morning, that has caused the fog.

"There's relatively little wind this morning, very light winds and they won't help shift the fog."

Mr Bennett said there were reports of the fog stretching down the WA coast as far south as Albany.

Pressure on Perth's roads was also increased by a technical fault on the Mandurah train line, forcing commuters to use replacement buses.

The issue has been resolved and trains are running again on the Mandurah line.