Thick fog affects flights in Brisbane

A think blanket of fog that's reduced visibility to 20 metres is affecting flights in and out of Brisbane.

Flights in and out of Brisbane airport are being delayed due to a thick fog covering the city. (AAP)

A blanket of heavy fog is affecting flights in and out of Brisbane airport.

Visibility is down to 20 metres at the airport and in the heart of the city, the weather bureau says.

The fog, which extends as far west as Roma, is expected to lift by about 9am but travellers are being warned to check with their airlines about cancellations and delays.

"There are flight delays and some diversions. Please check flight schedules for updates ... or contact your airline," Brisbane airport has tweeted.

The fog has also delayed CityCat ferry services for half an hour, in both directions.