Areas affected include London and south east England, Wales, west Midlands, south west England and north west England.
The alert, which will be in place until 8.30am this morning, warns of slower journey times and the possibility of delays to bus and train services.
Visibility may drop to below 200 metres in places, the Met Office stated.
The weather forecaster wrote: “Fog patches, some of them quite dense with visibility below 200 metres in places, will persist into the morning rush hour before lifting and clearing by mid-morning.
“Denser fog patches are probably more likely in rural areas and near rivers and lakes.”