The Met Office has issued a yellow warning for fog across large parts of the UK, including London and the south east.
The forecasting service said foggy conditions could cause transport disruption, including flight delays.
"Fog is expected to become increasingly widespread through Wednesday evening and overnight into Thursday. Slower journey times are possible and there is a chance of delays to flights," the Met Office said.
Fog patches are likely to form during the early part of Wednesday evening, becoming more widespread overnight with the visibility falling below 100 metres in places.
"Fog will thin slowly later on Thursday morning, although some patches may persist into the afternoon in the west of the warning area."