The images, captured by a National Police Air Service helicopter on Tuesday morning, show a blanket of fog covering the crossing linking England and Wales.
The fog appears to have been caused by a weather phenomenon known as temperature inversion.
This happens when a layer of air sinks towards the ground, drying and warming as it goes.
This creates a warm layer of air that acts as a “lid” and traps cooler air near the surface, according to the Met Office.
After the pictures were posted by the NPAS South West Twitter account, one motorist responded: “This was surreal, especially driving through it.”
Further images captured by Highways England cameras showed the bridge – which carries the M4 motorway – submerged in mist as the sun rises in the background.
Additional reporting by Press Association