A yellow warning for dense fog is in place across Berkshire and Buckinghamshire until 10am today.
Dense fog, with visibility below 100 metres at times, is expected to cause travel disruption during the morning rush hour.
Journey times may be longer, with delays possible for bus and train services.
Flights could also face delays or cancellations, while ferry travel may be affected.
People driving are advised to check road conditions before leaving and allow extra time for their journeys.
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Plans should be changed if needed, based on the latest travel information.
Drivers should make sure they know how to use fog lights and check that the lights are working before setting off.
Bus and train passengers should check updates from their travel companies and follow advice.
Fog will slowly thin, lift and clear later this morning in most places.
However, it may remain until this afternoon in some rural parts of eastern England.
The Met Office says the most extensive and disruptive fog will be during the morning rush hour.
The warning covers Berkshire, Buckinghamshire, Bracknell Forest, Milton Keynes, Oxfordshire, Reading, Slough, West Berkshire, Windsor and Maidenhead, and Wokingham.
It also includes Hampshire, Surrey and areas across London and south-east England.
Residents are urged to keep checking area forecasts because weather warnings can change.
Anyone travelling should leave extra time and consider amending plans if necessary.
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