Fog blanketing suburbs across the Perth metro area has triggered a warning to drivers.

Motorists are being warned to drive with additional caution on Perth roads Monday morning as fog is significantly reducing visibility.

Main Roads early Monday issued an alert stating there is fog over the road in “various suburbs”.

The organisation said drivers should “drive with caution” and “allow extra travel time”.

A combination of moisture in the air, clear sky and light wind overnight is to blame for the morning fog.

It comes ahead of a downpour expected for the city throughout the morning which is forecast to clear by midday.

Elsewhere on Perth roads, a hose in the right lanes on the Mitchell Freeway southbound at Warwick Road in Greenwood is slowing traffic.

Main Roads is warning drivers to “proceed with caution”.